What is the realistic accuracy & precision of typical consum

On Fri, 21 Jul 2017 22:05:13 -0000 (UTC), Mad Roger
<rogermadd@yahoo.com> wrote:

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.

True of the speedometer, but NOT of the odometer. The odometer
repeatabilty is as close to 100% as you will get even with a cable
driven odometer. (it is a directly geared measuring device with ZERO
vatiability - X number of cable turns per mile from the day it's made
till the day it is scrapped ( generally 1000 turns per mile, but some
older cars were 600 turns per mile, some motorcylses 1450, etc - but
they never change) With electronic speedos and odos (virtually all
cars today less than 15 years old) repeatability is almost 100%.
Accuracy CAN be very close to 100% too, as on most cars under 10 years
old today, the speedometer can be accurately reprogrammed to the tire
diameter so repeatability is only affected by tire wear (mabee 3/8
inch in 24 over the life of the tire)

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.

 

On Fri, 21 Jul 2017 22:25:04 +0000 (UTC), root <NoEMail@home.org>
wrote:

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.

Directly related? but not necessarily 100% linearly related High
manifold pressure (low vacuum) means heavy load which means poor
mileage. The reverse is also true - but calibrating vacuum to MPG is
virtually impossible with any level of accuracy. It WILL give you a
good, better, worse indication though. Keep the vacuum up and you will
get better mileage.

 

On Fri, 21 Jul 2017 23:13:15 -0000 (UTC), Mad Roger
<rogermadd@yahoo.com> wrote:

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?

How many ingels can sit on the tip if a pin???
That's about how ridiculous this whole discussion is getting

 

[rickman]

clare@snyder.on.ca wrote on 7/21/2017 9:47 PM:

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.

Nearly all my driving is on secondary highways so I pretty much am driving
at pretty optimal speeds for mileage although there are some traffic lights,
they tend to be miles between stops. I have developed fuel efficient habits
so I nearly always squeeze every last MPG on my trips. I have a manual, so
I slip it out of gear and coast to lights and nearly always accelerate
gently. I leave a lot of room to the car in front so I can ease up on the
gas rather than hit the brakes. I think I am doing about as well as can be
expected all the time, so my mileage seldom varies unless I do more city
driving. High 19 or low 20 MPG, very consistent.

--

Rick C

 

[rickman]

Mad Roger wrote on 7/21/2017 7:13 PM:

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

I don't know what you mean. I have checked my odometer against the markers
on the highway as well as against my GPS (I think the highway markers are
more accurate than the GPS). It is spot on with the current tires to 1% or
better. I had some larger tires at one point and it made the odometer read
a bit low, also the speedometer.

BTW, someone said something about one being accurate meant the other was
accurate and that is not necessarily true. My speedometer is mechanical and
so has a separate calibration factor. With the present tires it reads a bit
high, about 1 to 1.5 MPH at highway speeds. That one is harder to calibrate
than the odometer (which is pretty much on point) because it is hard to
maintain a constant speed for long enough to get an accurate reading even
with the cruise control. But with lots of readings I am pretty confident
these numbers are right.

So my odometer is accurate and precise.


> + A pumpmeter of 20.25 gallons is likely relatively accurate & precise

Of course it is. States inspect them at some point.


> + Matching fuel level in the tank isn't even close to accurate nor precise

I don't agree. I let the pump click off and then continue to pump for a
number of more clicks until it cuts off immediately. I always need to run
at least another fifteen miles before I am home so that is better part of a
gallon burned so I don't need to worry about the gas warming up and running
out of the tank. I believe this makes for very consistent fill ups.

My MPG results pretty well show the consistency of my measures.

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.

You know what happens when you assume...

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.

I think one time in nearly 20 years I got 22 MPG. I think I can count on my
fingers the times I got 21 MPG. These days with nearly all my driving on
the highway it is much less than 1 in 20 fills that I see less than 19 or
even 19.5 MPG. It is nearly always just under or just over 20 MPG, more
just under :-( If I were the dancing type I would have a little happy
dance when it actually is over 20 MPG, lol. It makes my day.

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?

I think the consistency of my MPG readings show how well each of these can
be measured. As you say, the pump is going to be dead on. Other than scale
error which can be calibrated out the odometer will be very good. Filling
your tank can be good as well. It's not like they design gas tanks to have
air pockets.

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?

You don't need to know any of this specifically. You just need to measure
your fuel mileage and measure the accuracy and precision of the results.
Why do you care which of the three has what specific degrees of accuracy and
precision? You care about the accuracy and precision in the result and you
can measure that. Remember there are other factors as well that actually
impact your MPG from tank to tank. They will show up when trying to measure
any one influence so might as well calibrate them in too.

--

Rick C

 

[rbowman]

On 07/21/2017 07:55 PM, dpb wrote:

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

When you consider how the old speedometers worked it's amazing they came
anywhere close to reality. I had a '60 Plymouth where the speedometer
looked like a red bar progressing across a horizontal display rather
than the usual needle. The guts were a tube about a foot long and an
inch and a half in diameter suspended in bearings and loaded with a
spiral spring. The mechanical cable from the tailshaft of the
transmission tweaked the tube with each revolution via a magnetic link.
It was an analog integrator with the spring controlling the tube's rotation.

The standard dial type was the same principal but the Chrysler engineers
went out of their way to be weird. That was also the era of the
pushbutton Torqueflite tranny and left handed lugnuts on one side.

A lot of modern speedometers are just as bizarre converting a perfectly
good digital pulse train to an analog voltage to drive a dial rather
than going straight digital.

But now

 

[rbowman]

On 07/21/2017 07:47 PM, clare@snyder.on.ca wrote:

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.

I'm a fairly economical driver but on longish trips I'm more concerned
with getting there. 80 mph guarantees the fuel economy is going into the
dumpster.

 

[pfjw@aol.com]

Once again: This horse is dead, skinned, flensed, tanned, jerked and dried. And whatever life it has clung to for these many repetitive, redundant and often ridiculous posts is based on the essential confusion between and conflation of "Accuracy" and "Precision". These do not mean the same thing, never have the same application, and seldom are on even parallel tracks when used properly.

When dealing with furlongs per bale, sufficient accuracy may be had using no more than 20% of the fingers of one normal human hand and the track markers. Miles per gallon, and kilometers per liter are quite similar, requiring only second-grade arithmetic to solve within repeatable limits. Full stop.

Peter Wieck
Melrose Park, PA

 

[rickman]

rbowman wrote on 7/22/2017 1:29 AM:

On 07/21/2017 07:47 PM, clare@snyder.on.ca wrote:
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.

I'm a fairly economical driver but on longish trips I'm more concerned with
getting there. 80 mph guarantees the fuel economy is going into the dumpster.

I forgot, I can tell the difference in fuel economy by driving 65 MPH rather
than 60. Driving at 65 very much (only about 1/3 of my trip allows that)
will assure that I only get 19 mpg rather than pushing 20.

There is a 10 mile stretch with only one traffic light and a posted speed
limit of 45 MPH. If I can get up to 50 so I'm solid in fifth gear my
mileage rocks.

--

Rick C

 

[Colonel Edmund J. Burke]

On 7/20/2017 5:08 AM, Mad Roger 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?

Stuff like that doesn't concern a man of my means, but in a pinch I could measure my mileage knowing the actual distance to my nearest Denny's, America's Favorite Diner.
This is the "meat and potatoes" of the argument. All else is just fluff.

 

clare@snyder.on.ca posted for all of us...

On Fri, 21 Jul 2017 23:13:15 -0000 (UTC), Mad Roger
rogermadd@yahoo.com> wrote:

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?
How many ingels can sit on the tip if a pin???
That's about how ridiculous this whole discussion is getting

+1

--
Tekkie

 

[rbowman]

On 07/22/2017 07:22 AM, rickman wrote:

rbowman wrote on 7/22/2017 1:29 AM:
On 07/21/2017 07:47 PM, clare@snyder.on.ca wrote:
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.

I'm a fairly economical driver but on longish trips I'm more concerned
with
getting there. 80 mph guarantees the fuel economy is going into the
dumpster.

I forgot, I can tell the difference in fuel economy by driving 65 MPH
rather than 60. Driving at 65 very much (only about 1/3 of my trip
allows that) will assure that I only get 19 mpg rather than pushing 20.

There is a 10 mile stretch with only one traffic light and a posted
speed limit of 45 MPH. If I can get up to 50 so I'm solid in fifth gear
my mileage rocks.

I should look at the instantaneous readouts versus mph to see if the mpg
falls off gradually or if there is an efficiency sweet spot around
65-70. Except for around the cities the interstate speed limit in this
and some of the adjoining states is 80. Drive 65 at your own risk.

 

[Ed Pawlowski]

On 7/22/2017 1:38 PM, rbowman wrote:

I should look at the instantaneous readouts versus mph to see if the mpg
falls off gradually or if there is an efficiency sweet spot around
65-70. Except for around the cities the interstate speed limit in this
and some of the adjoining states is 80. Drive 65 at your own risk.

I tried that one day on a flat stretch so there would be little
variance. This was on my regular trip to work. Speed limit is 65. One
day I did 70, the next 65, then at 60 is was dicey, the next day I tried
55 for about 30 seconds and decided not to risk my life.

I forget the details, but 60 was better than 70 by a couple of mpg.
Problem is, I prefer driving 75. If I could get away with it I'd go 85+
but don't want to pay the fines.

 

[Mad Roger]

On Sat, 22 Jul 2017 00:46:50 -0400,
rickman wrote:

> So my odometer is accurate and precise.

I understand you because you're exactly the type of person that I had in
mind when I asked the question in the first place.

I don't know what you mean. I have checked my odometer against the markers
on the highway as well as against my GPS (I think the highway markers are
more accurate than the GPS). It is spot on with the current tires to 1% or
better.

Does your tripmeter have a decimal place and digits after that decimal
place?

> My speedometer is mechanical and so has a separate calibration factor.

The speedometer example was only brought in to point out that the vain hope
that averages result in better "accuracy" is patently false.

Mom-and-pop type of people actually believe that a speedometer reads even
close to accurately - and worse - some here propose the vain notion that
the more readings they take, somehow (magically?) the more accurate the
results will be.

A speedometer that reads high isn't going to result in more accurate
calculations even if you do a billion test runs.

+ A pumpmeter of 20.25 gallons is likely relatively accurate & precise

Of course it is. States inspect them at some point.

You don't seem to understand what accuracy and precision even mean.
Haven't you taken even one science lab course?

+ Matching fuel level in the tank isn't even close to accurate nor precise

I don't agree. I let the pump click off and then continue to pump for a
number of more clicks until it cuts off immediately.

I'm not at all surprised about your concept of the fuel-level estimation,
and, in fact, you're exactly the mom-and-pop type person I was talking
about when I opened the thread.

I understand you.

I always need to run
at least another fifteen miles before I am home so that is better part of a
gallon burned so I don't need to worry about the gas warming up and running
out of the tank. I believe this makes for very consistent fill ups.

I'm sure you do believe that.

> My MPG results pretty well show the consistency of my measures.

I'm sure your MPG results support any theory you want them to support.
I believe you.

> You know what happens when you assume...

You don't know how funny that statement was to me when I just read it now.

> I see less than 19 or even 19.5 MPG.

I bet you see that decimal place even though it's not in the tripmeter
estimation nor in the filllevel estimation.

You see, I understand you because you're the type of person I had in mind
when I asked the question.

I think the consistency of my MPG readings show how well each of these can
be measured.

I'm sure you do.

> As you say, the pump is going to be dead on.

Whoa! I never said the pump was "dead on" and anyone reading this thread
who thinks I think the pump is "dead on" would have completely
misunderstood everything else I said.

All I said was that the inaccuracies and imprecisions in the pump reading
are likely better than the otherwise astoundingly huge imprecision in the
fuel-fill level estimation and in the lesser inaccuracy of the tripmeter
estimation.

Other than scale
error which can be calibrated out the odometer will be very good.

Define "very good" please.

> Filling your tank can be good as well.

I'm sure you believe that filling the tank is "accurate" since you
calculate 19.5 miles per gallon and not something like 19.5 rounded up to
20 and then the error taken into account such that it's more likely
anywhere between 19 and 21 mpg than it is 19.5 mpg.

> It's not like they design gas tanks to have air pockets.

Actually, they do have air pockets.
Those air pockets change in size based on temperature & pressure & fill
level.

Even the fuel changes in density based on those parameters.

> You don't need to know any of this specifically.

Of course I don't. 19.5 mpg is all I need to know.
And if I change "something" which results in 19.7mpg, then of course, that
something was the cause. I understand. I really do.

Why do you care which of the three has what specific degrees of accuracy and
precision?

I care because when I do a calculation, my assumption is that 19.5mpg is
actually something closer to 19 to 21 mpg than it is to 19.5.

If the "change" I'm measuring is within that margin of error, then I can't
say anything about what that "change" was.

And, more importantly, neither can you.
Which is the entire point after all.

 

[dpb]

On 07/21/2017 8:30 PM, dpb wrote:

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%

....

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.

I got curious myself on what the numbers revealed and looked at the NIST
numbers again.

I computed an empirical cdf and compared it to normal...statistics from
the 20,036 observations are below:

=cdfplot(x);
s

s =
min: -50
max: 146
mean: -0.0788
std: 3.7681
median: 0
mode: 0
>>

I then compared to normal on the same plot and as outlined above
N(mean,std) is too long-tailed on both ends in comparison. It turns out
that N(mean,std/1.5) is pretty close on both tails to about the +/- 6 point.


Anyway, from the above it's simple enough to get some pretty good
estimates of what pump volume errors one might expect...the table below
is from the empirical cdf NIST data...

P error(in^3)/5Gal error(%)
0.001 -22 -1.82
0.005 -9 -0.78
0.010 -8 -0.69
0.025 -6 -0.52
0.050 -5 -0.43
0.250 -2 -0.17
0.500 0 0
0.750 2 0.17
0.900 4 0.34
0.950 5 0.43
0.975 6 0.52
0.990 7 0.60
0.995 10 0.86
0.999 22 1.82

From the above, one can conclude the pump metering error small for all
except the extreme outlier pumps.

--

 

On Sat, 22 Jul 2017 23:42:25 -0000 (UTC), Mad Roger
<rogermadd@yahoo.com> wrote:

On Sat, 22 Jul 2017 00:46:50 -0400,
rickman wrote:

So my odometer is accurate and precise.

I understand you because you're exactly the type of person that I had in
mind when I asked the question in the first place.

I don't know what you mean. I have checked my odometer against the markers
on the highway as well as against my GPS (I think the highway markers are
more accurate than the GPS). It is spot on with the current tires to 1% or
better.

Does your tripmeter have a decimal place and digits after that decimal
place?

My speedometer is mechanical and so has a separate calibration factor.

The speedometer example was only brought in to point out that the vain hope
that averages result in better "accuracy" is patently false.

Mom-and-pop type of people actually believe that a speedometer reads even
close to accurately - and worse - some here propose the vain notion that
the more readings they take, somehow (magically?) the more accurate the
results will be.

A speedometer that reads high isn't going to result in more accurate
calculations even if you do a billion test runs.

+ A pumpmeter of 20.25 gallons is likely relatively accurate & precise

Of course it is. States inspect them at some point.

You don't seem to understand what accuracy and precision even mean.
Haven't you taken even one science lab course?

+ Matching fuel level in the tank isn't even close to accurate nor precise

I don't agree. I let the pump click off and then continue to pump for a
number of more clicks until it cuts off immediately.

I'm not at all surprised about your concept of the fuel-level estimation,
and, in fact, you're exactly the mom-and-pop type person I was talking
about when I opened the thread.

I understand you.

I always need to run
at least another fifteen miles before I am home so that is better part of a
gallon burned so I don't need to worry about the gas warming up and running
out of the tank. I believe this makes for very consistent fill ups.

I'm sure you do believe that.

My MPG results pretty well show the consistency of my measures.

I'm sure your MPG results support any theory you want them to support.
I believe you.

You know what happens when you assume...

You don't know how funny that statement was to me when I just read it now.

I see less than 19 or even 19.5 MPG.

I bet you see that decimal place even though it's not in the tripmeter
estimation nor in the filllevel estimation.

You see, I understand you because you're the type of person I had in mind
when I asked the question.

I think the consistency of my MPG readings show how well each of these can
be measured.

I'm sure you do.

As you say, the pump is going to be dead on.

Whoa! I never said the pump was "dead on" and anyone reading this thread
who thinks I think the pump is "dead on" would have completely
misunderstood everything else I said.

All I said was that the inaccuracies and imprecisions in the pump reading
are likely better than the otherwise astoundingly huge imprecision in the
fuel-fill level estimation and in the lesser inaccuracy of the tripmeter
estimation.

Other than scale
error which can be calibrated out the odometer will be very good.

Define "very good" please.

Filling your tank can be good as well.

I'm sure you believe that filling the tank is "accurate" since you
calculate 19.5 miles per gallon and not something like 19.5 rounded up to
20 and then the error taken into account such that it's more likely
anywhere between 19 and 21 mpg than it is 19.5 mpg.

It's not like they design gas tanks to have air pockets.

Actually, they do have air pockets.
Those air pockets change in size based on temperature & pressure & fill
level.

Even the fuel changes in density based on those parameters.

You don't need to know any of this specifically.

Of course I don't. 19.5 mpg is all I need to know.
And if I change "something" which results in 19.7mpg, then of course, that
something was the cause. I understand. I really do.

Why do you care which of the three has what specific degrees of accuracy and
precision?

I care because when I do a calculation, my assumption is that 19.5mpg is
actually something closer to 19 to 21 mpg than it is to 19.5.

If the "change" I'm measuring is within that margin of error, then I can't
say anything about what that "change" was.

And, more importantly, neither can you.
Which is the entire point after all.

The man is right You are wrong. You ASS U ME too much - and at the
risk of insulting the few GOOD engineers on the list, you OBVIOUISLY
are an "engineer", but not one I'd hire for a job. The job would come
in WAY over budget, WAY late, and would need to be completely redone
by techitians and technologists at great cost, or to save time and
money, completely decommissioned and scrapped - starting over with
someone who knew what thet were doing, and how to do it - engineer or
not.

 

a whole lot of crap snipped

I care because when I do a calculation, my assumption is that 19.5mpg is
actually something closer to 19 to 21 mpg than it is to 19.5.

If the "change" I'm measuring is within that margin of error, then I can't
say anything about what that "change" was.

And, more importantly, neither can you.
Which is the entire point after all.

Roger, me lad - you wouldn't happen to be a britiah trained engineer,
now, would you?? In what discipline of engineering?

 

[Mad Roger]

On Sat, 22 Jul 2017 19:44:26 -0500,
dpb wrote:

I got curious myself on what the numbers revealed and looked at the NIST
numbers again.

I computed an empirical cdf and compared it to normal...statistics from
the 20,036 observations are below:

[2 quoted lines suppressed]
s =
min: -50
max: 146
mean: -0.0788
std: 3.7681
median: 0
mode: 0
[1 quoted line suppressed]

I then compared to normal on the same plot and as outlined above
N(mean,std) is too long-tailed on both ends in comparison. It turns out
that N(mean,std/1.5) is pretty close on both tails to about the +/- 6 point.


Anyway, from the above it's simple enough to get some pretty good
estimates of what pump volume errors one might expect...the table below
is from the empirical cdf NIST data...

P error(in^3)/5Gal error(%)
0.001 -22 -1.82
0.005 -9 -0.78
0.010 -8 -0.69
0.025 -6 -0.52
0.050 -5 -0.43
0.250 -2 -0.17
0.500 0 0
0.750 2 0.17
0.900 4 0.34
0.950 5 0.43
0.975 6 0.52
0.990 7 0.60
0.995 10 0.86
0.999 22 1.82

From the above, one can conclude the pump metering error small for all
except the extreme outlier pumps.

I love that you are the only one quoting actual numbers and not pulling
them out of your butt to answer the question!

But your numbers confuse me because they seem to be in cubic inches.
You also mentioned that metric pumps are more accurate, but that's
impossible, simply because the pump is as accurate as the pump can get,
which, we can assume, is a mechanical thing (and not a metric thing).

All you're saying is that a liter is four times smaller than a gallon so
the error is four times less for a given liter versus a given gallon but
that's not saying it's more accurate. It's just saying the volume is less
so the resulting error is less.

Anyways, can you just summarize what the error is for a typical USA pump in
gallons?

For a typical 20-gallon fill, how many gallons off can reality be, plus or
minus from the indicated reading on the pumpmeter?

 

[rbowman]

On 07/22/2017 11:52 AM, Ed Pawlowski wrote:

On 7/22/2017 1:38 PM, rbowman wrote:



I should look at the instantaneous readouts versus mph to see if the
mpg falls off gradually or if there is an efficiency sweet spot around
65-70. Except for around the cities the interstate speed limit in this
and some of the adjoining states is 80. Drive 65 at your own risk.


I tried that one day on a flat stretch so there would be little
variance. This was on my regular trip to work. Speed limit is 65. One
day I did 70, the next 65, then at 60 is was dicey, the next day I tried
55 for about 30 seconds and decided not to risk my life.

I forget the details, but 60 was better than 70 by a couple of mpg.
Problem is, I prefer driving 75. If I could get away with it I'd go 85+
but don't want to pay the fines.

At under 70 my car usually is in the 35 mpg + range; at 80, it is more
like 32. I get even better mileage in Oregon with its 55 mph speed
limit. I also get bored out of my mind. There isn't a whole lot of
anything between Ontario and Bend but I figure as soon as I get up to a
decent speed a OSP cruiser will materialize from the sagebrush.

That stupid speed limit is the least of Oregon's problems.

 

[rbowman]

On 07/22/2017 07:14 PM, clare@snyder.on.ca wrote:

a whole lot of crap snipped

I care because when I do a calculation, my assumption is that 19.5mpg is
actually something closer to 19 to 21 mpg than it is to 19.5.

If the "change" I'm measuring is within that margin of error, then I can't
say anything about what that "change" was.

And, more importantly, neither can you.
Which is the entire point after all.
Roger, me lad - you wouldn't happen to be a britiah trained engineer,
now, would you?? In what discipline of engineering?

Engineering Management, I'm thinking.