What is the realistic accuracy & precision of typical consum

[dpb]

On 07/21/2017 2:05 PM, Mad Roger wrote:
....

Remember, the resulting accuracy can't possibly be better than the least
accurate measurement.

On a _point_ estimate, yes.

The point I'm making is that it is the _total_ fuel consumed over the
total distance; the changes in hitting the target level on a
tank-by-tank basis goes away for all excepting the last tank as it
doesn't matter in the total. So, if you miss by 0.1 gal on the one
tank, yeah, that roughly will translate to 0.1 on the mpg number. But,
over the 9 tanks prior to the tenth and last, it doesn't matter; it was
all used and so the 0.1 gal error on the last is only a tenth of the
size on the overall as it was on the first.

So, over a time, you can get quite precise estimates this way.

As noted, the bias in odometer calibration is a bias, yes, but presuming
there's not a reason it is getting worse with time it's not compounding,
it just makes a percentage difference in the computed result.

--

 

[Mad Roger]

On Fri, 21 Jul 2017 13:31:01 -0500,
Vic Smith wrote:

Even the EPA's $360,000 machine only claims plus or minus 2% of the
indicared reading. I can't find where I got the notion that a mom and pop
can't possibly get closer than about 4% with a tripmeter/pumpmeter mpg
calculation - but I'm still seeking those numbers as we speak.

The EPA doesn't have the time to do accurate MPG numbers Plus/Minus 2% is good enough for
the EPA. I'm sure my "mom and pop" MPG number are more accurate than that.
But so what? The MPG I get depend on the driving circumstances.
For instance, I've measured my MPG on a number of cars on 3000 mile round trips to Florida.
I can tell you the EXACT total MPG I got on those 3000 mile trips because I carefully noted
the exact metered amount of gas I used, and I verified the odometer accuracy using mile
markers. The only real useful thing that gives me is my MPG for the entire trip.
That includes local traffic when getting off the highway, and my travels at my destination.
But I know my approximate MPG at steady highway speed because I sometimes do tank to tank
calculations by filling to the filler tube. That too is an EXACT calculation, but is still
only approximate MPG because maybe the terrain and weather may vary.
So before you ask about "accurate MPG" you have to define what that is.

We're trying to compare a MPG *change* between two situations:
+ Calculation before the change (say, smaller tire/wheel diameter)
+ Calculation after the change (say, larger time/wheel diameter)

If the change itself causes, say, a 1 mpg difference, but if our
measurement accuracy is, say, plus or minus 1 mpg, then we'll never see a
measurable difference between the two test runs.

Even if we run ten thousand test runs, we'll still never see a
statistically valid difference, even though the 1 mpg difference is
actually there.

We can't measure any better than our accuracy and repeatability allows.

The factors, I think, are accuracy, precision, repeatability, and, since
multiplication/division is involved, each offset worsens the results.

Without answering these questions, nobody, yes, not even you, can say you
have an "exact" number, and, I posit, that you can't even get remotely
close to exact, using the standard mom-and-pop tripmeter/pumpmeter method.

+ Tripmeter accuracy is what in the average car over a 300-mile tank?
+ Owners ability to "match" the previous level of fuel is what?
+ Gas station pump reading accuracy is what?

 

[Mad Roger]

On Fri, 21 Jul 2017 14:20:43 -0500,
dpb wrote:

On a _point_ estimate, yes.

The point I'm making is that it is the _total_ fuel consumed over the
total distance; the changes in hitting the target level on a
tank-by-tank basis goes away for all excepting the last tank as it
doesn't matter in the total. So, if you miss by 0.1 gal on the one
tank, yeah, that roughly will translate to 0.1 on the mpg number. But,
over the 9 tanks prior to the tenth and last, it doesn't matter; it was
all used and so the 0.1 gal error on the last is only a tenth of the
size on the overall as it was on the first.

So, over a time, you can get quite precise estimates this way.

As noted, the bias in odometer calibration is a bias, yes, but presuming
there's not a reason it is getting worse with time it's not compounding,
it just makes a percentage difference in the computed result.

Your multiple-runs argument only holds water for both random accuracy and
random precision, but not if one is random and the other is not.

For example, I think it's well known that most speedometers read high
*most* of the time (at least that's my understanding - but I could look
that up if you question that assertion).

Assuming that assertion is close to correct, let's say they read high by
about 5% accuracy most the time (just to make a point), where the precision
is about plus or minus 1%.

Notice the accuracy is *always* high while the precision is random around a
set point.

http://www.chem.tamu.edu/class/fyp/mathrev/mr-sigfg.html
Accuracy: how closely a measured value agrees with the correct value.
Precision: how closely individual measurements agree with each other.

If the speedo reads high by 5% all the time, whether you measure your speed
once or if you measure your speed a billion times, you'll never any closer
to the right speed than 5% plus or minus 1%.

In repeatability, the gauge may give you different figures within + or - 1%
of that 5%, which is only to say that the speed will be consistently
reading from 4% to 6% higher than the actual speed.

But a billion test runs won't get you any better than that, all of which
are at least 4% off from the "correct" measurement (in the example).

My point is that a billion test runs only randomizes that which is random.

 

[root]

Mad Roger <rogermadd@yahoo.com> wrote:

We're trying to compare a MPG *change* between two situations:
+ Calculation before the change (say, smaller tire/wheel diameter)
+ Calculation after the change (say, larger time/wheel diameter)

Putting different size wheels on the rear will affect the mileage
measurement apart from the mpg, so you will have to correct
the miles measurement before computing mpg. Smaller wheels => higher
miles for the same real distance. You will have to take into
account how you drive with the wheel change. If you maintain
the same real speed for smaller wheels your engine will be
turning over faster than before. Driving at the same speedometer
speed with smaller wheels reduces the load on the engine.

As a somewhat off-topic point, manifold vacuum is directly related
to instantaneous mpg. It is relatively easy to install a vacuum
gauge in the driver's compartment.

 

[rickman]

Ralph Mowery wrote on 7/20/2017 10:57 AM:

In article <okqbo5$c17$1@dont-email.me>, csr684@NOTyahoo.com says...

clare@snyder.on.ca wrote:
On Thu, 20 Jul 2017 12:08:33 -0000 (UTC), Mad Roger
rogermadd@yahoo.com> wrote:

What is a realistic accuracy & precision of typical MPG measurements when
measured by the consumer using the typical method of dividing their
tripmeter miles by the gas-pump gallons during fillup?
Repeatabilty is terrible. Accuracy can be pretty good over multiple
tanks. Can be pretty good even on single tanks IF there is a way to
ensure the tank is always filled to EXACTLY the same point (like a
level in the fill - tube, with the vehicle parked at EXACTLY the same
place for each fill-up). Relying on the auto-shutoff of the pump can
cause variance of several liters per fillup.

Vehicle position or the auto shut off point won't make any difference.
You read the amount of fuel you pumped off the pump itself. The only
real issue is odometer accuracy. That can vary with tire size variations
and factory calibration.

If you just do it one time, you can not be sure you put in the same
ammount of fuel that was taken out.

If you keep a running total of the ammount of miles and fuel over
several tank fulls , the ammount of fuel will sort of average out.

Say you park so the back of the car is up hill and you fill the tank.
Go a number of miles and fill up again. This time the back of the car
is down hill. You may burn out 15 gallons, but only put back in 14
gallons. Ot it could be the other way around and you burn 14 gallons,
but only put back in 13 gallons. From tank to tank full there could be
a large variation. Over many tanks, the variation will average out to a
lessor error.
After say 10 tanks used you only have to contend with one or two errors
caused by the exact ammount of fuel put in the tank. Probably just the
last tank full would be where the error would come in. So instead of 1
gallon of error like the example above, you would have about .1 gallon
of error if the pumps are correct, which they should be.

You obviously don't check your mileage very often. I do and seldom see even
1 MPG difference. I get high 19 or low 20 MPG on 19 out of 20 tanks. I
think the idea of uneven filling of the tank is a red herring. I can't
remember the last time I saw a gas station on a slope.

Bottom line is try it and see. I expect the major factor in MPG variation
is actual MPG variation from driving a different mix of town and highway
driving.

--

Rick C

 

[dpb]

On 07/21/2017 5:05 PM, Mad Roger wrote:
....

But a billion test runs won't get you any better than that, all of which
are at least 4% off from the "correct" measurement (in the example).

My point is that a billion test runs only randomizes that which is random.

We'd already thrown the mileage calibration bias out as being simply
that. It can be compensated for by comparison over set measured course
and recording the offset. Red herring for the discussion.

The point I'm making is that it doesn't matter at all about whether
there's any random error in the fillup on individual tanks at all on the
intermediary answers--yes, they may have some fluctuation owing to
inconsistent fillup, but one can assume the pump is accurate since
they're checked by the State weights and measures folks on a regular
basis. So, all the fuel that went in went out in accumulating the miles
and it didn't matter how much went in on each individual measurement at
all in the end--it's the total. Only that random error on the final
fillup when you make the calculation at the end does that error enter in
-- and it becomes quite small by then in comparison to the total.

And, if one computes the mean of the billion trials, the error in the
mean is quite small even if the random error in each trial is sizable.

--

 

[Mad Roger]

On Fri, 21 Jul 2017 18:39:37 -0400,
rickman wrote:

You obviously don't check your mileage very often. I do and seldom see even
1 MPG difference. I get high 19 or low 20 MPG on 19 out of 20 tanks. I
think the idea of uneven filling of the tank is a red herring. I can't
remember the last time I saw a gas station on a slope.

I don't think I've ever seen a gas station on a slope.
However ...
The errors in the calculation stem from errors that nobody seems to know
what they are, which means nobody knows what they're talking about.

Assuming the tripmeter/pumpmeter calcultion is the method used,
+ A tripmeter of 300 miles is neither accurate nor precise
+ A pumpmeter of 20.25 gallons is likely relatively accurate & precise
+ Matching fuel level in the tank isn't even close to accurate nor precise

Any one measurement (either miles or gallons alone) can only be as accurate
and precise as the worst measurement, while the miles/gallons calculation
compounds those inaccuracies and imprecisions (in loss of sig figs).

I think most of us would probably assume the pumpmeter is the most accurate
and the most precise, but the other two measurements aren't even close to
accurate or precise.

What matters is how accurate & precise is a 300 mile tripmeter reading?
And how accurate and precise is the match to the previous fuel level?

I posit that the best you can do, overall, after running the calculation,
is something like plus or minus about 1 mile per gallon such that 20 mpg is
actually anywhere from 19 to 21 miles per gallon actual.

What I'm seeking is data that tells us the three main questions that must
be answered for anyone to say that my hypothesis is even close to being
right or wrong:

+ How accurate & precise is a reading of 300 miles on a typical tripmeter?
+ How accurate & precise is a reading of 20.25 gallons on a gas pump?
+ How accurate & precise is the matching of the prior fuel level done?

No calculation can do better than the worst measurement, and worse, errors
compound when you multiply or divide.

Bottom line is try it and see. I expect the major factor in MPG variation
is actual MPG variation from driving a different mix of town and highway
driving.

But that's the kind of things we're looking for, which is why the minimum
number of calculations possible is two, since you have to have a "before"
situation and an "after" situation.

For example, if the change that you are testing causes about 1 mile per
gallon decrease in fuel economy overall (but which isn't linear), but if
your calculations are no better than plus or minus 1 mile per gallon in
accuracy or precision, then you'll never even see the very real difference
because it will be unmeasurable given the plus or minus 1 mile per gallon
typical accuracy and precision that I posit the typical mom-and-pop
tripmeter/pumpmeter calculation provides.

But there's no sense in talking about *any* of that, if we don't know the
answer to these three questions.
+ How accurate & precise is a reading of 300 miles on a typical tripmeter?
+ How accurate & precise is a reading of 20.25 gallons on a gas pump?
+ How accurate & precise is the matching of the prior fuel level?

 

[Mad Roger]

On Fri, 21 Jul 2017 18:39:37 -0400,
rickman wrote:

You obviously don't check your mileage very often. I do and seldom see even
1 MPG difference. I get high 19 or low 20 MPG on 19 out of 20 tanks. I
think the idea of uneven filling of the tank is a red herring. I can't
remember the last time I saw a gas station on a slope.

I don't think I've ever seen a gas station on a slope.
However ...
The errors in the calculation stem from errors that nobody seems to know
what they are, which means nobody knows what they're talking about.

Assuming the tripmeter/pumpmeter calcultion is the method used,
+ A tripmeter of 300 miles is neither accurate nor precise
+ A pumpmeter of 20.25 gallons is likely relatively accurate & precise
+ Matching fuel level in the tank isn't even close to accurate nor precise

Any one measurement (either miles or gallons alone) can only be as accurate
and precise as the worst measurement, while the miles/gallons calculation
compounds those inaccuracies and imprecisions (in loss of sig figs).

I think most of us would probably assume the pumpmeter is the most accurate
and the most precise, but the other two measurements aren't even close to
accurate or precise.

What matters is how accurate & precise is a 300 mile tripmeter reading?
And how accurate and precise is the match to the previous fuel level?

I posit that the best you can do, overall, after running the calculation,
is something like plus or minus about 1 mile per gallon such that 20 mpg is
actually anywhere from 19 to 21 miles per gallon actual.

What I'm seeking is data that tells us the three main questions that must
be answered for anyone to say that my hypothesis is even close to being
right or wrong:

+ How accurate & precise is a reading of 300 miles on a typical tripmeter?
+ How accurate & precise is a reading of 20.25 gallons on a gas pump?
+ How accurate & precise is the matching of the prior fuel level done?

No calculation can do better than the worst measurement, and worse, errors
compound when you multiply or divide.

Bottom line is try it and see. I expect the major factor in MPG variation
is actual MPG variation from driving a different mix of town and highway
driving.

But that's the kind of things we're looking for, which is why the minimum
number of calculations possible is two, since you have to have a "before"
situation and an "after" situation.

For example, if the change that you are testing causes about 1 mile per
gallon decrease in fuel economy overall (but which isn't linear), but if
your calculations are no better than plus or minus 1 mile per gallon in
accuracy or precision, then you'll never even see the very real difference
because it will be unmeasurable given the plus or minus 1 mile per gallon
typical accuracy and precision that I posit the typical mom-and-pop
tripmeter/pumpmeter calculation provides.

But there's no sense in talking about *any* of that, if we don't know the
answer to these three questions.
+ How accurate & precise is a reading of 300 miles on a typical tripmeter?
+ How accurate & precise is a reading of 20.25 gallons on a gas pump?
+ How accurate & precise is the matching of the prior fuel level?

 

[Vic Smith]

On Fri, 21 Jul 2017 21:51:42 -0000 (UTC), Mad Roger <rogermadd@yahoo.com> wrote:

On Fri, 21 Jul 2017 13:31:01 -0500,
Vic Smith wrote:


So before you ask about "accurate MPG" you have to define what that is.

We're trying to compare a MPG *change* between two situations:
+ Calculation before the change (say, smaller tire/wheel diameter)
+ Calculation after the change (say, larger time/wheel diameter)

If the change itself causes, say, a 1 mpg difference, but if our
measurement accuracy is, say, plus or minus 1 mpg, then we'll never see a
measurable difference between the two test runs.

Under just moderately controlled conditions, 1 MPG is actually a significant amount, and
would be easily detectable using your mom and pop method, assuming the mom and pop
competently applied the method.

Even if we run ten thousand test runs, we'll still never see a
statistically valid difference, even though the 1 mpg difference is
actually there.

We can't measure any better than our accuracy and repeatability allows.

The factors, I think, are accuracy, precision, repeatability, and, since
multiplication/division is involved, each offset worsens the results.

Without answering these questions, nobody, yes, not even you, can say you
have an "exact" number, and, I posit, that you can't even get remotely
close to exact, using the standard mom-and-pop tripmeter/pumpmeter method.

+ Tripmeter accuracy is what in the average car over a 300-mile tank?
+ Owners ability to "match" the previous level of fuel is what?
+ Gas station pump reading accuracy is what?

I never used the tripmeter for MPG, because I never bothered testing them with mile
markers.
Matching gas level is trivial - and it only has to done at the beginning and end of the
trip.
Gas station pumps - I assume they are accurate, and can't control that anyway.
I'm confident that my measurements are accurate to within .1 MPG.
Because I don't care about .135867 on the total. I round it down to .1
That's pretty "exact." Repeatability is meaningless when measuring MPG, unless you're
driving on a covered track, with a temperature controlled environment.
On every trip the MPG can vary.
BTW, I don't disagree that perfect measurement of MPG in unattainable. Perfection is
impossible even under lab conditions.
But you too easily discount "mom and pop" MPG calculations.

 

[Mad Roger]

On Fri, 21 Jul 2017 18:05:52 -0500,
dpb wrote:

We'd already thrown the mileage calibration bias out as being simply
that. It can be compensated for by comparison over set measured course
and recording the offset.

I agree with you that the tripmeter calculation is inaccurate to some
degree, for which there are ways of "calibrating" the equipment.

> Red herring for the discussion.

The answer to the question depends on only 3 factors, I think.

Given these three factors are critical to answer the question, I think
everyone is talking out of their ass (including me) if they can't answer
these three questions to validate their own thought process:

+ How accurate & precise is a reading of 300 miles on a your tripmeter?
+ How accurate & precise is a reading of 20.25 gallons on the gas pump?
+ How accurate & precise is the matching of the prior fuel level?

I posit both the tripmeter and the previous-fill-level measurements suck.
How much to they suck?

I don't know.

I would not be surprised if they suck so badly that the end result is a
calculation which is plus or minus 1 mile per gallon in either accuracy or
repeatability.

The point I'm making is that it doesn't matter at all about whether
there's any random error in the fillup on individual tanks at all on the
intermediary answers--yes, they may have some fluctuation owing to
inconsistent fillup, but one can assume the pump is accurate since
they're checked by the State weights and measures folks on a regular
basis.

While it will be useful to know what the accuracy and precision
(repeatability) of the pump is, I think we can all assume that the pump is
within something like (at least) plus or minus a few percent of what it
reads.

But that number can be accurate to a billionth of a gallon, and it still
would be meaningless if the fill level was off by plus or minus a gallon
because the accuracy of any one measurement is only as good as the worst
measurement and the accuracy of the final calculation (when multiplication
adn division are involved) compounds inaccuracies.

Anyway, all the words are moot if we don't know the answer to 3 questions:
+ How accurate & precise is a reading of 300 miles on a your tripmeter?
+ How accurate & precise is a reading of 20.25 gallons on the gas pump?
+ How accurate & precise is the matching of the prior fuel level?

So, all the fuel that went in went out in accumulating the miles
and it didn't matter how much went in on each individual measurement at
all in the end--it's the total. Only that random error on the final
fillup when you make the calculation at the end does that error enter in
-- and it becomes quite small by then in comparison to the total.

Am I correct to understand that you are saying if you go only 300 miles on
one tank, then the fill-level inaccuracy is (say) plus or minus 1 gallon
per tank; but if you go 3,000 miles (obviously on multiple tanks), that the
fill-level inaccuracy is one tenth of that plus or minus one gallon per
tank?

And, if one computes the mean of the billion trials, the error in the
mean is quite small even if the random error in each trial is sizable.

As long as the error is random (i.e., in both directions of the true
answer).

 

[Mad Roger]

On Fri, 21 Jul 2017 18:22:08 -0500,
Vic Smith wrote:

> I'm confident that my measurements are accurate to within .1 MPG.

Assuming one tank is about 300 miles and about 20.25 gallons read on any
one pump, on a long trip of ten times that, you still can't be confident of
that 1/10th of a mpg unless you know the answer to these questions.

+ How accurate & precise is the combined reading of 3000 miles on your tripmeter?
+ How accurate & precise is the combined reading of 202.50 gallons on the 10 gas pumps?
+ How accurate & precise is the matching of the 1st & last fuel levels?

Remember the exammple of the speedometer, where it's *always* going to be a
few percent wrong, even if you drove a billion miles to try to "randomize"
out the errors.

 

[dpb]

On 07/21/2017 6:13 PM, Mad Roger wrote:
....

> + How accurate& precise is a reading of 300 miles on a typical tripmeter?

For a specific vehicle, it really doesn't matter to determine _changes_
in mpg for a given test condition (unless, of course, you're futzing
with the tire size in which case that would have to be compensated for
but is doable to pretty precise number by knowing the tire profiles or
simply doing the "measured mile" computation.

> + How accurate& precise is a reading of 20.25 gallons on a gas pump?

NIST tolerance is 6 in^3 in a 5 gal measure. AFAIK that's what all
state W&M departments use for their tolerance. A NIST document of
20,000 tested meters showed 0-mean normally distributed discrepancies at
about 90% bounds on the +/-6 number. The 6/5gal --> ~0.5%

> + How accurate& precise is the matching of the prior fuel level done?

That's entirely dependent upon the tester -- in older vehicles without
the emissions control folderol on the gas tank filler spout it was
pretty easy to be quite precise if one were trying. Now it would take
some doing, but I suspect if really cared, one could manage to get
pretty close to the same height.

No calculation can do better than the worst measurement, and worse, errors
compound when you multiply or divide.
....

But properly designed 'spearmints can cause cancellation of many sources
often and besides the simple "combination of error" the math of the
numbers also enters into the relative magnitude of the error on the
final result.

In this case, for example, the second can be effectively eliminated or
reduced significantly by simply taking multiple runs...then the actual
level on each intermediary measurement is immaterial because whether it
was high or low on any given case, the total amount of fuel is the
denominator and so the fractional error in it is much smaller owing to
the same presumed error in the last measurement.

--


--

 

[dpb]

On 07/21/2017 7:02 PM, dpb wrote:
....

+ How accurate& precise is a reading of 20.25 gallons on a gas pump?

NIST tolerance is 6 in^3 in a 5 gal measure. AFAIK that's what all state
W&M departments use for their tolerance. A NIST document of 20,000
tested meters showed 0-mean normally distributed discrepancies at about
90% bounds on the +/-6 number. The 6/5gal --> ~0.5%

+ How accurate& precise is the matching of the prior fuel level done?
....

And remember that is the "shut 'er down" tolerance, not the average...as
noted, the most probable based on the NIST sample was in the +/-0 bin (<1).

I didn't quite recognize what the figure was yet when first looked at it
and had closed the link when I realized the significance so don't have
the actual numbers at hand...but the +/-6 number was quite a way out on
the tails of the distribution altho I don't know just precisely the
tails percentages.

And, actually while the report used "normal" in discussing the
distribution, it really wasn't normal as in bell-shaped, it was
symmetric and zero-mean, but the tail in each direction dropped off more
as hyperbolic than a normal--hence the tail percentages would actually
by somewhat lower than a real normal of same mean, standard deviation.

--

 

[dpb]

On 07/21/2017 6:30 PM, Mad Roger wrote:
....

As long as the error is random (i.e., in both directions of the true
answer).

What do you not understand about "random"?

And the mean is still the mean, whether it is zero or not.

--

 

On Fri, 21 Jul 2017 12:42:08 -0500, dpb <none@non.net> wrote:

On 07/21/2017 11:51 AM, Mad Roger wrote:
...

Most people have a tripmeter reading and a gas pumpmeter reading.
Where they fill the tank and reset the tripmeter before driving away.

I can't find any reliable source that says what the accuracy or
repeatability of that mom-and-pop tripmeter/pumpmeter calculation, but
basic logic dictates that the errors compound such that there is likely
(IMHO) no way to get anywhere near decimal-point accuracy, and worse,
probably plus or minus 1 mpg is the closest anyone can get in terms of
repeatability and precision.
...

Why do errors compound in your view?

And, it depends on what you mean in terms of accuracy -- in terms of
absolute one needs to know the calibration error of the odometer; most
folks are satisfied to just assume it's close enough for the purpose.

If you look at simply a single fillup, it's not unreasonable to expect a
few tenths of a gallon difference between the first fillup level and the
subsequent; if you try it on shorter distances than a full tank then the
fractional error goes up.

OTOH, if one keeps track over longer periods of multiple fillups and
take some care to use the same filling pattern and only fills up after
using near the full tank capacity, then over time plus/minus targets
_will_ tend to cancel out and I have no qualms in believing a relative
performance number in the 0.1 mpg can be determined.

As noted, I've done this on long trips a number of times (generally on
first trip or so with a new vehicle, either actually new (rare) or (most
often) new to me) just to see how it compared with previous and have had
quite good comparisons on recent ones with the computer-computed
results. These would be over total distances of 1500 to 2000 miles, not
just 20 miles test runs.

With my scanguage on my Ranger the calculated MPG and the MPG figured
out by me using a calculator and fuel volume vs mileage is generally
pretty darn close. If the ScanGuage says they injectors have passed
13.7 gallons, my fillup is generally somewhere within .1 to .2
gallons. The speedo and GPS are within less than 1 kph on speed at
100kph (62Mph), and the odo within about the same. This is after
making corrections over many tanks for the fuel volume adjustments.
Neither of my ancient machines has a built in "trip computer"

 

On Fri, 21 Jul 2017 14:02:38 -0400, rickman <gnuarm@gmail.com> wrote:

Vic Smith wrote on 7/20/2017 8:29 AM:
On Thu, 20 Jul 2017 12:08:33 -0000 (UTC), Mad Roger <rogermadd@yahoo.com> wrote:

What is a realistic accuracy & precision of typical MPG measurements when
measured by the consumer using the typical method of dividing their
tripmeter miles by the gas-pump gallons during fillup?

Close to 100% accuracy if done right. I've done it on long trips. But MPG will vary
depending on terrain, weather, wind direction, stop-and-go traffic, etc.
So if you want "true" MPG for your car, you have to do it for the life of the car.
Once you do it initially, it's kind of pointless to do again except to satisfy your
curiosity.

+1

I measure my gas mileage on every fillup. I get 19 to 20 MPG every fill
unless I do a lot of around town driving. Very consistent. I watch it to
see if it drops off which would mean something is wrong.

Occaisionally on a longish trip I'll see how well I can drive for
economy - to see if I can better the last time I did that trip. This
is generally over pretty close to a full tank - and small differences
in driving technique can make a HUGE difference. So can a small change
in route.Or a difference in the wind. I've registered a good 25%
difference in mileage between 2 trips, both trying to squeeze the last
foot out of a liter of fuel. Round trip averages out the difference in
altitude.

 

[dpb]

On 07/21/2017 8:41 PM, clare@snyder.on.ca wrote:
....

With my scanguage on my Ranger the calculated MPG and the MPG figured
out by me using a calculator and fuel volume vs mileage is generally
pretty darn close. If the ScanGuage says they injectors have passed
13.7 gallons, my fillup is generally somewhere within .1 to .2
gallons. The speedo and GPS are within less than 1 kph on speed at
100kph (62Mph), and the odo within about the same. This is after
making corrections over many tanks for the fuel volume adjustments.
Neither of my ancient machines has a built in "trip computer"

I find all that perfectly believable...the speedo on the Buick I noticed
on the last trip to NM was deadnuts on the wife's GPS. I presume if
that's the case the odo would be, too, altho I never actually checked on
it specifically.

Instrumentation is pretty good and pretty cheap to get pretty good for
ordinary measurements any more...electronics is a wunnerful help in many
ways.

As I've turned into old fogey, I've come to rely on the 'puter for such
info more than would've cared at 20. There's just only so many times
one really wants to get down on the ground and measure tire pressure
after 70... There's getting to be a lot of fluff besides the useful,
but the basic stuff is helpful.

--

 

On Fri, 21 Jul 2017 13:31:01 -0500, Vic Smith
<thismailautodeleted@comcast.net> wrote:

On Fri, 21 Jul 2017 16:51:17 -0000 (UTC), Mad Roger <rogermadd@yahoo.com> wrote:



Even the EPA's $360,000 machine only claims plus or minus 2% of the
indicared reading. I can't find where I got the notion that a mom and pop
can't possibly get closer than about 4% with a tripmeter/pumpmeter mpg
calculation - but I'm still seeking those numbers as we speak.

The EPA doesn't have the time to do accurate MPG numbers Plus/Minus 2% is good enough for
the EPA. I'm sure my "mom and pop" MPG number are more accurate than that.
But so what? The MPG I get depend on the driving circumstances.
For instance, I've measured my MPG on a number of cars on 3000 mile round trips to Florida.
I can tell you the EXACT total MPG I got on those 3000 mile trips because I carefully noted
the exact metered amount of gas I used, and I verified the odometer accuracy using mile
markers. The only real useful thing that gives me is my MPG for the entire trip.
That includes local traffic when getting off the highway, and my travels at my destination.
But I know my approximate MPG at steady highway speed because I sometimes do tank to tank
calculations by filling to the filler tube. That too is an EXACT calculation, but is still
only approximate MPG because maybe the terrain and weather may vary.
So before you ask about "accurate MPG" you have to define what that is.

We'll never get that - he admits he's an "engineer"
1) Someone who gets excited obout things most other people don't care
about
2) Someone who solves a problem you didn't know you had, in a way you
don't understand.
3) The optimist sees the glass as half full. The pessimist sees the
glass as half empty. The engineer sees the glass as twice as big as it
needs to be


Q: What is the definition of an engineer?
A: Someone who solves a problem you didn't know you had in a way you
don't understand.

Q: When does a person decide to become an engineer?
A: When he realizes he doesn't have the charisma to be an undertaker.

Q: How can you tell an extroverted engineer?
A: When he talks to you, he looks at your shoes instead of his own.

Q: Why did the engineers cross the road?
A: Because they looked in the file and that's what they did last
year.

Q: How do you drive an engineer completely insane?
A: Tie him to a chair, stand in front of him, and fold up a road map
the wrong way.

Real Engineers consider themselves well dressed if their socks match
Real Engineers buy their spouses a set of matched screw- drivers for
their birthday.
Real Engineers wear moustaches or beards for "efficiency". Not
because they're lazy.
Real engineers have a non-technical vocabulary of 800 words.
Real Engineers think a "biting wit" is their fox terrier.
Real Engineers know the second law of thermodynamics - but not their
own shirt size.
Real Engineers say "It's 70 degrees Fahrenheit, 25 degrees Celcius,
and 298 degrees Kelvin" and all you say is "Isn't it a nice day"
Real Engineers don't find the above at all funny.

 

On Fri, 21 Jul 2017 19:05:32 -0000 (UTC), Mad Roger
<rogermadd@yahoo.com> wrote:

On Fri, 21 Jul 2017 12:42:08 -0500,
dpb wrote:

Why do errors compound in your view?

It's like a chain is no stronger than the weakest link.

No calculated result can be better than the worse inaccuracy.

And, it depends on what you mean in terms of accuracy -- in terms of
absolute one needs to know the calibration error of the odometer; most
folks are satisfied to just assume it's close enough for the purpose.

Accuracy, precision, and sigfigs are standard terms:
http://www.chem.tamu.edu/class/fyp/mathrev/mr-sigfg.html
Accuracy: how closely a measured value agrees with the correct value.
Precision: how closely individual measurements agree with each other.
Sigfigs: accuracy is no better than the least accurate measurement.

By way of off-the-cuff example, if the accuracy of the odometer is to the
billionth of a mile and the accuracy of the pump gallons is to the
billionth of a gallon, but the accuracy of the fillup is plus or minus one
gallon, then the resulting mathematical (division or multiplication)
accuracy can be no better than plus or minus one gallon.

If you look at simply a single fillup, it's not unreasonable to expect a
few tenths of a gallon difference between the first fillup level and the
subsequent; if you try it on shorter distances than a full tank then the
fractional error goes up.

A single fillup will never suffice.

We're trying to compare a MPG *change* between two situations, so, by
definition, there _must_ be (at the very least!) /two/ separate
calculations.
+ Calculation before the change (say, smaller tire/wheel diameter)
+ Calculation after the change (say, larger time/wheel diameter)

OTOH, if one keeps track over longer periods of multiple fillups and
take some care to use the same filling pattern and only fills up after
using near the full tank capacity, then over time plus/minus targets
_will_ tend to cancel out and I have no qualms in believing a relative
performance number in the 0.1 mpg can be determined.

That's not necessarily true, because it depends on the understimations
balancing out the overestimations, but I'm not going to quibble that more
calculations done over time are likely going to randomize the precision and
accuracy fluctuations over time.

Actually it is - because you only make ONE calculation. The
measurement accuracies do not change. The precision gets better,
because you are not using numerous mileage measurements that can only
be accurate to the closest 10th of a mile or KM (more accurate in the
metric system becuase a KM is smaller than a mile) The precision is
now 1/10km over 5000 km instead of 1/10 kn over 500, ten times.

Your accuracy on the fuel used is to the 10th of a liter (or gallon
if you are a Yank)- again more accurate with the metric system because
liters are smaller than gallons. So your accuracy is to the closest
10th of a liter 5 times - and the accuracy of the fillup is only the
closest you can get it ONCE instead of 5 times.

Accuracy of fuel used will be, at the very most, 5X 1/10th liter more
- that's plus half a liter over to -0 liter under over 365 liters at
20 miles per US Gallon that's within better than 1.5% (1.369) at the
outside.(assuming the calibration of the pumps is accurate - pumps are
calibration tested on a more or less regular basis - when I was "in
the business" we were still running imperial gallons for the most part
- the pumps were inspected and certified accurate to within 1/10
gallon in 5 gallons ( the closest the meter could read) IIRC when the
switch to metric was made, it was 1/10 of a liter in 20 liters (the
size of the test container remained virtually the same) so accuracy
improved by roughly a factor of 4.

If I kept track of the fuel mileage on my vehicle over a period of
50,000 km, the accumulated average fuel economy could be easily
calculated to within that percentage of error. ( I used to do that
when I ran a vehicle log for business purposes)

While I will not quibble with your statement (because I essentially agree
with you), I can point out that your speedometer can be consistently wrong
in the same direction in either precision or accuracy, in which case it's
*not* going to balance out over time. It will be consistently wrong, over
time.

Or consistently right. If you KNOW the accuracy of the speedo, it is a
simple mathematical correction to achieve accuracy. The speedo may
vary in accuracy because it is an inductive coupling device on a
mechanical speedo, while the ODO will not vary as it is a direct
geared connection to the driven wheels. With electronic speedos and
odos, their calibration does NOT change. The only vatiance is tire
wear ( aproxematelt 3/8 inch difference in diameter of a , say, 24
inch diameter tire, over it's lifetime - and that wear is pretty
linear - so it is not rocket science to work in a correction for that
too if you want to be a very anal engineer.

But, let's not quibble about that because we both can assume that, for our
purposes, the randomization of measurement results will be half the time
underestimating and the other half the time overestimating - such that they
could balance out.

Or they could not - better to eliminate the randomization, or account
for it in calculating accuracy.

As noted, I've done this on long trips a number of times (generally on
first trip or so with a new vehicle, either actually new (rare) or (most
often) new to me) just to see how it compared with previous and have had
quite good comparisons on recent ones with the computer-computed
results. These would be over total distances of 1500 to 2000 miles, not
just 20 miles test runs.

I've averaged it over 240,000 miles - - -

Nobody yet, and even not me, has supported a claim for any better accuracy
than my presumed plus or minus one mile per gallon using the standard
mom-and-pop test of dividing the number of miles driven based on the
tripmeter reading by the pump indication of gallons used to fill back up to
a presumed similar previous starting point of amount of fuel consumed.

I'm sure I could claim accuracy to closer than 1 MPG, but what good
would it do over 240,000 miles??????? (and how could you prove me
wrong?)

For COMPARISON testing, accuracy is not important - only precision
and repeatability. On my electric conversion I could compare one type
of tire to another by driving a given distance and route on one set,
measure the watt hours of charge used, and compare to a different set
of tires over the same route under the same conditions and KNOW how
much better "mileage" I would get with one tire over the other.

Modifying the tune on my '63 Valiant, or a customer's Celica, or
whatever - I could do a "before and after" run of 5 miles with my
calibration can and know, to the ounce, how much more or less fuel was
consumed over the same route, Using the "official" fuel mile tester I
could measure to the cc over a half liter - that's an accuravy of 1
part in 500, or 0.2 percent. If I had a "rolling road" I could repeat
the drive cycle accurately too - but I only had access to that at
trade school (a chassis dynamometer) Not as easy to do today with fuel
injected cars - but dynos are a LOT more common today than they were
40 to 45 years ago . . . and more programmable. If you know the cd of
a vehicle today, a computer simulation can run a vehicle over a
virtual course, correcting for ambient wind, changes in elevation,
accelleration (knowing mass of the car) -every conceivable condition
- to make direct case to case comparisons EXREMELY accurate. (and fuel
measurement technology has advanced so it's very simple to very
accurately neasure the amount of fuel consumed as well - and also get
very accurate measurements of instantaneous consumption - and with
strain guages even know EXACTLY how much horsepower is being delivered
to the road to figure out specific fuel consumption -

All stuff you "engineers" should understand.

Remember, the resulting accuracy can't possibly be better than the least
accurate measurement.

Which can vary from no better than a SWAG to pretty darn close, even
for the "mom and pop" or "hobyist" to EXTREMELY accurate for the
engineer.

 

On Fri, 21 Jul 2017 21:51:42 -0000 (UTC), Mad Roger
<rogermadd@yahoo.com> wrote:

On Fri, 21 Jul 2017 13:31:01 -0500,
Vic Smith wrote:

Even the EPA's $360,000 machine only claims plus or minus 2% of the
indicared reading. I can't find where I got the notion that a mom and pop
can't possibly get closer than about 4% with a tripmeter/pumpmeter mpg
calculation - but I'm still seeking those numbers as we speak.

The EPA doesn't have the time to do accurate MPG numbers Plus/Minus 2% is good enough for
the EPA. I'm sure my "mom and pop" MPG number are more accurate than that.
But so what? The MPG I get depend on the driving circumstances.
For instance, I've measured my MPG on a number of cars on 3000 mile round trips to Florida.
I can tell you the EXACT total MPG I got on those 3000 mile trips because I carefully noted
the exact metered amount of gas I used, and I verified the odometer accuracy using mile
markers. The only real useful thing that gives me is my MPG for the entire trip.
That includes local traffic when getting off the highway, and my travels at my destination.
But I know my approximate MPG at steady highway speed because I sometimes do tank to tank
calculations by filling to the filler tube. That too is an EXACT calculation, but is still
only approximate MPG because maybe the terrain and weather may vary.
So before you ask about "accurate MPG" you have to define what that is.

We're trying to compare a MPG *change* between two situations:
+ Calculation before the change (say, smaller tire/wheel diameter)
+ Calculation after the change (say, larger time/wheel diameter)

If the change itself causes, say, a 1 mpg difference, but if our
measurement accuracy is, say, plus or minus 1 mpg, then we'll never see a
measurable difference between the two test runs.

Even if we run ten thousand test runs, we'll still never see a
statistically valid difference, even though the 1 mpg difference is
actually there.

We can't measure any better than our accuracy and repeatability allows.

The factors, I think, are accuracy, precision, repeatability, and, since
multiplication/division is involved, each offset worsens the results.

Without answering these questions, nobody, yes, not even you, can say you
have an "exact" number, and, I posit, that you can't even get remotely
close to exact, using the standard mom-and-pop tripmeter/pumpmeter method.

+ Tripmeter accuracy is what in the average car over a 300-mile tank?

Whatever it is off, repeatability will be very close to 100%.
300 miles, repeatabilty to within less than 1/10 mile, or .2%
>+ Owners ability to "match" the previous level of fuel is what?
If he's as smart as the average fifth grader, within less than a cup
at the same station, in the same spot, Lets nake it within a pint -
that is 1/8 gallon in 15 - call it .8% on 15 gallons - - - so at
LEAST within 1%
>+ Gas station pump reading accuracy is what?
Again, repeatability within well under 1/10 gallon per fill (15
gallons) - about .6%??
Accuracy doesn't mean anything if the refill is at the same pump as
the initial fill.
Again, if you are in the "metric world" instead of the USA, accuracy
improves by a factor of almost 4 on the fuel measurements, and 1.6 on
the distance.


All of this accuracy can be accomplishes with NO special equipment.

Where the REAL fun comes in is duplicating the "drive cycle" - for the
average driver, on the average road, "virtually impossible". If you
have a closed course, and you are a VERY good and consistent driver, -
perhaps you can get within a REASONABLE approxemation (repeatability
over 5 runs, perhaps within less than 5%) - in open traffic, you'l be
doing VERY good to get repeatability better than within 10% on a short
run - over 300 miles, averaged over 5 runs (total 1500 miles) mabee
within 1% on open road, or 5% running with traffic.
Repeatability gets a LOT better over a short distance where fewer
variables are involved. (Start from the gas station, accellerate to
30MPH in 2 blocks, enter the "expressway" and accellerate to 60mph by
the first exit, maintain 60MPH to the 5th exit, slow down and exit the
highway, stop. pull away from stop and re-enter the highway going the
opposite direction, accellerate to 60 mph as you merge with traffic,
decellerate from the 4th exit to the third exit, return to the gas
station
Repeat.
Repeat
Repeat.

That kind of driving can be very repeatable.

Fill with gas, Drive 3 blocks to the highway, drive 100 miles to "the
city" Drive 5 miles across the city to restaurant, sports arena, or
place of work, then return

Repeat
Not much chance the 2 trips will be anywhere close. (I have had
variances of more than 25% between 2 trips between Waterloo and Barrie
Ontario over the same route, at the same time of day, and day of the
week (and even the same month) - (even when the overall travel time
was very close to the same for the round trip). generally WITHIN 10%
IS PRETTY DOABLE - AND AT 20mpg THAT'S +/- 1 mpg at best

If you are in open country with very little traffic, repeatability
gets MUCH better.
Driving back and forth from Flagstaff to Jeddito Arizona you could
likely repeat within well less than 1/4 MPG in the middle of the week
- or from Enid to Chickasha Oklahoma - or from Saskatoon to
Regina.(very little traffic variability)