What is the realistic accuracy & precision of typical consum

[rbowman]

On 07/22/2017 08:40 PM, Mad Roger wrote:

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

http://www.npr.org/templates/story/story.php?storyId=5304258

The article talks about Washington but most states have a similar
protocol. Pump 5 gallons of gas. 1 gallon is 231 cubic inches, so that
is 1155 cubic inches. The volume must be within 6 cubic inches or
roughly 0.5%. I'll let you do the math for 20 gallons.

http://billingsgazette.com/news/state-and-regional/montana/customers-rarely-shorted-at-the-gas-pump/article_3849a455-6151-515e-ae6a-2d65351736b4.html

Montana uses the same test. Note that he estimates 2 to 3% of the pumps
fail and have to be repaired and also says with normal wear the pumps
tend to dispense more than stated but some may dispense less. That's
where averaging over a number of tanks comes in unless you fill up at
the same pump at the same station every time. I certainly don't.

 

On Sun, 23 Jul 2017 02:40:10 -0000 (UTC), Mad Roger
<rogermadd@yahoo.com> wrote:

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 are the engineer, son of physics majors - figure it out!!!
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).

You fail to grasp the simple fact that a tenth of a liter is a whole
lot less than a tenth of a gallon???? Accuracy of READING the pump is
therefore about 4 times more accurate with a metric pump, because your
read error of +.1/-0 units is based on the much smaller unit.

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.

and your engineer's understanding of accuracy does not equate to a
smaller error?????????

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

As good as Less than 1/10 of a percent according to the information
quoted, with a very few as bad as 1.82%. An american gallon is 128
fluid ounces, so 1.82% of 128 ounces is 2.23 ounces maximum error,
+/1, with most being within .5%, or 0.64 ounces per gallon

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

The poorest pump checked in that data would be +/- 44.6 oz per 20
gallon tank - the average about +/- 12 ounces.
ASS U MEing the error is randomly distributed,around zero, your
chances of the error being anywhere CLOSE to even the 12 ounces is so
small as to be virtually insignificant unless you always used the same
pump - in which case it is totally immaterial if used for comparative
purposes.

For an engineer, you sure have a poor grasp of the concepts.

 

[Bill Vanek]

On Sat, 22 Jul 2017 22:12:26 -0600, rbowman <bowman@montana.com>
wrote:

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.

Where is their limit 55?

 

[rbowman]

On 07/22/2017 10:45 PM, Bill Vanek wrote:

On Sat, 22 Jul 2017 22:12:26 -0600, rbowman <bowman@montana.com
wrote:

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.

Where is their limit 55?

The last time I was there US20, US395, and other 2 lane roads in eastern
Oregon. Apparently the raised it to 65 in March of 2016 but are rolling
it back in some places.

http://www.oregonlive.com/commuting/index.ssf/2016/06/oregon_rolls_back_speed_limit.html

but according to this the limit is now 70 on rural roads:

http://www.speed-limits.com/oregon.htm

70 on Rt. 20 would make a lot more sense if that is indeed what it is
now. I'm not planning to check it out personally though.

 

[Vic Smith]

On Sat, 22 Jul 2017 21:10:41 -0400, clare@snyder.on.ca wrote:

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?

snip

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.

You know, this guy has a hard-on against "non-engineers" measuring their MPG.
Rickman above told him he uses his odometer, then he goes on about tripmeters.
I answered his main complaints in another post. That exchange went like this:

">+ 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."

His response to me totally ignored those responses, and he posed the same questions again!
Then, for some reason, he stated talking about speedometers.
He's a troll.

 

[rickman]

rbowman wrote on 7/22/2017 1:38 PM:

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.

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.

--

Rick C

 

[rickman]

Mad Roger wrote on 7/22/2017 7:42 PM:

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?

I've never seen a trip odometer that didn't have tenths of a mile.

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.

Only because averages don't impact the effect of limited accuracy, averaging
mitigates the effect of limited precision. But both precision and accuracy
impact the error in any one reading.

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?

I think you are missing something. What you replied do does not in any way
indicate a limited understanding of precision and accuracy. But affect each
measurement taken. An inspection measurement will require the combination
of accuracy and precision in that measurement be within some limit. What do
you expect them to do, take dozens of measurements? There are economic
considerations, especially since this is about economics anyway. It is to
prevent excess profits from being made by shortchanging the customers.

+ 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.

Not sure what that means. What I am doing by repeatedly topping off is to
reach the point where the fuel in the filler neck is right at the nozzle so
it won't run anymore, but rather cuts off immediately. This results in a
very consistent fill level.

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.

I think my consistent mileage measurements support my conclusions.

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 seem to be doubting my results. Are you suggesting I am fudging my data?

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 seem obsessed with evaluating the resulting MPG measurement even though
you can't put numbers on the accuracy of the parameters that impact the MPG
errors. If you can't come up with numbers, your ideas are of no value. But
that doesn't mean the errors in my MPG measurements aren't as they appear to
be.

Actually, I do have numbers for the parameters. I know the mileage to a
fraction of a mile (even though a tenth mile out of 400 is far more accurate
than anything else involved) and I have no reason to doubt the pump giving
me 20.0 gal when it says 20.0 gal. I don't fill up at the same pump each
time so if some were off it would show up and I'd be able to identify which
pumps were inaccurate.

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.

You keep saying this without indicating what you mean.

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.

Lol! You see, I understand you because you're the type of person I had in
mind when I made that comment.

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

Define "very good" please.

Have done, 0.1 mile over 100 miles has been calibrated... actually, it was
much better than 0.1 mile since I can interpolate the analog dial. I don't
drive that stretch of road anymore, so I can't calibrate 100.0 miles
continuously anymore or I would.

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.

Sorry, your sentence doesn't make sense to me. Can you construct it properly?

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.

If what you say is true, why is it I have only seen 21 mpg a very, very few
times in the 20 years I have been checking my mileage? If what you are
saying is true, I should see a much wider variation in measurements than I
see. As I have said, 95% of the time I get between 19.5 and 20.5 mpg or
within a 4% range (+-2%). It's actually even tighter than that. It's more
like 19.7 to 20.2 mpg but I can't say just how often.

--

Rick C

 

[pfjw@aol.com]

KEEERIST!!!

Miles driven per gallon, kilometers driven per liter, furlongs realized per bale. Pretty basic. And within "R-E-A-L-I-S-T-I-C" tolerances, may be calculated using 2nd grade arithmetic.

However! There are various factors that will affect results:

Speed driven
Distance driven
Style of driving
Type of vehicle
Load on vehicle
Condition of vehicle
Terrain
Type of tires
Condition of tires
Tire inflation
Condition of bearings & CV joints (if any)
Condition of suspension

So, we are able to make a snapshot of any given trip. And an average of several trips, that will give us a practical expectation of consumption based on our style in our car in its present condition. Not to be confused with an actual and accurate description of consumption - as that not only can, but *W*I*L*L* change with any change in the above parameters, and likely several others not enumerated.

We are discussing CARS as they are used EVERY DAY. We are not discussing neurosurgery, rocket science, disease vectors nor anything else other than very broad-brush stuff.

Per Deacon Mushrat, 2,619 1/2 can dance on the head of a pin - so that is now settled science.

Yikes!

Peter Wieck
Melrose Park, PA

 

[root]

rickman <gnuarm@gmail.com> wrote:

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.

It is true that air resistance goes up a square of the speed, but
the power requirement, and the corresponding rate of fuel consumption,
goes up as the cube. Work=force*distance, Power=force*speed.

 

Vic Smith posted for all of us...

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."

His response to me totally ignored those responses, and he posed the same questions again!
Then, for some reason, he stated talking about speedometers.
He's a troll.

Exactly what I have been posting. This guy is the valve stem thread, bead
breaker, etc troll.

--
Tekkie

 

[pfjw@aol.com]

Y'all are feeding the troll by responding at any level pasts 2nd grade arithmetic. Full Stop.

Peter Wieck
Melrose Park, PA

 

[rickman]

root wrote on 7/24/2017 1:00 PM:

rickman <gnuarm@gmail.com> wrote:

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.


It is true that air resistance goes up a square of the speed, but
the power requirement, and the corresponding rate of fuel consumption,
goes up as the cube. Work=force*distance, Power=force*speed.

You are right that the horsepower requirement goes with the cube. But, that
doesn't impact the gas mileage. Since you are traveling faster you drive
for a shorter time, so that extra factor in power cancels out. No?

--

Rick C

 

On Mon, 24 Jul 2017 19:23:40 -0400, rickman <gnuarm@gmail.com> wrote:

root wrote on 7/24/2017 1:00 PM:
rickman <gnuarm@gmail.com> wrote:

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.


It is true that air resistance goes up a square of the speed, but
the power requirement, and the corresponding rate of fuel consumption,
goes up as the cube. Work=force*distance, Power=force*speed.

You are right that the horsepower requirement goes with the cube. But, that
doesn't impact the gas mileage. Since you are traveling faster you drive
for a shorter time, so that extra factor in power cancels out. No?

No, because the speed doubling takes only half the time, but 4 time
the power. Not necessarilly 4 times the fuel, because the engine may
be "on the cam" at the higher speed, running more efficiently.

An example of this was the 1975 Toyota Celica GT. With the 1975
gearing, it was actually most efficient at 80MPH in 5th, as long as
you didn't have to change speed or pass anyone. (I got 52MPG at just
over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
morning back in 1979-ish.

Didn't work on the 1976 model - same body (and engine) but different
gearing

 

[rickman]

clare@snyder.on.ca wrote on 7/24/2017 9:46 PM:

On Mon, 24 Jul 2017 19:23:40 -0400, rickman <gnuarm@gmail.com> wrote:

root wrote on 7/24/2017 1:00 PM:
rickman <gnuarm@gmail.com> wrote:

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.


It is true that air resistance goes up a square of the speed, but
the power requirement, and the corresponding rate of fuel consumption,
goes up as the cube. Work=force*distance, Power=force*speed.

You are right that the horsepower requirement goes with the cube. But, that
doesn't impact the gas mileage. Since you are traveling faster you drive
for a shorter time, so that extra factor in power cancels out. No?
No, because the speed doubling takes only half the time, but 4 time
the power. Not necessarilly 4 times the fuel, because the engine may
be "on the cam" at the higher speed, running more efficiently.

An example of this was the 1975 Toyota Celica GT. With the 1975
gearing, it was actually most efficient at 80MPH in 5th, as long as
you didn't have to change speed or pass anyone. (I got 52MPG at just
over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
morning back in 1979-ish.

Didn't work on the 1976 model - same body (and engine) but different
gearing

What was the lowest speed you could use 5th gear in the 75 car?

--

Rick C

 

On Mon, 24 Jul 2017 22:32:54 -0400, rickman <gnuarm@gmail.com> wrote:

clare@snyder.on.ca wrote on 7/24/2017 9:46 PM:
On Mon, 24 Jul 2017 19:23:40 -0400, rickman <gnuarm@gmail.com> wrote:

root wrote on 7/24/2017 1:00 PM:
rickman <gnuarm@gmail.com> wrote:

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.


It is true that air resistance goes up a square of the speed, but
the power requirement, and the corresponding rate of fuel consumption,
goes up as the cube. Work=force*distance, Power=force*speed.

You are right that the horsepower requirement goes with the cube. But, that
doesn't impact the gas mileage. Since you are traveling faster you drive
for a shorter time, so that extra factor in power cancels out. No?
No, because the speed doubling takes only half the time, but 4 time
the power. Not necessarilly 4 times the fuel, because the engine may
be "on the cam" at the higher speed, running more efficiently.

An example of this was the 1975 Toyota Celica GT. With the 1975
gearing, it was actually most efficient at 80MPH in 5th, as long as
you didn't have to change speed or pass anyone. (I got 52MPG at just
over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
morning back in 1979-ish.

Didn't work on the 1976 model - same body (and engine) but different
gearing

What was the lowest speed you could use 5th gear in the 75 car?

Can't remember for sure, but it was a DOG at 60mph - requiresd a
downshift to get anywhere. I think hey geared the 75 GT the same as
the 4 speed. I know I was shocked by the mileage on that trip - going
out to Kingston to pit crew for Taisto Heinonnen, "The Flying Fynn"
and Tom Burgess on the Twin Lakes Rally. Crewsd for him on the Tall
Pines and the Blossom too.

I was offered his backup Celica Team car in 1980 when we finished
rallying in the navigational rallye series (After finishing 1st,
second and third in 3 years we were no longer elligible) and our R12
was not adequate to run competetively in the performance series but I
decided to quit while I was ahead, since I was getting married.

 

[rbowman]

On 07/24/2017 07:46 PM, clare@snyder.on.ca wrote:

An example of this was the 1975 Toyota Celica GT. With the 1975
gearing, it was actually most efficient at 80MPH in 5th, as long as
you didn't have to change speed or pass anyone. (I got 52MPG at just
over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
morning back in 1979-ish.

I had the misfortune to own a '71 Audi when the 55 mph national speed
limit went into effect. The German engineers thought 55 was a very brief
period on your way to cruising speed not a speed you'd try to drive.