What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)

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.

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.

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

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.

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

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

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.

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.

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

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.

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?

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.

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They're not the only seemingly bizarre place--between Clayton and Springer is 100 mi of open country with either 55 (or _maybe_ 60) that makes no common sense at all...

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Well, yes, as said before the NIST standard for compliance testing is a metering error of You also mentioned that metric pumps are more accurate, ...

_I_ said nothing about metric anywhere during the thread. Another respondent pointed out that a liter, being smaller than a gallon, when metered to the same tenth of a unit as the gallon will be a smaller absolute error than in gallons. Seems fairly obvious...

The above data showed that the most probable error was 0 (actually less than +/-1 ci since data are reported to nearest whole number). I mentioned multiple times already and its in the table the distribution was symmetrical and the mode and median were both 0. The mean is just under 0.1 ci (-0.08) so there'd be a good place to start for just a random pump taken from the population of pumps.

Take your choice of how conservative you want to be or how likely it is to be of that magnitude--that's why I gave the ecdf data--you can choose the appropriate number for the particular use.

20*0.018 --> 0.36 gal would encompass 99.9% of all observed pumps on either the over- or under-dispensing side; the likelihood finding an operational pump of that drastic a metering error would be only 0.1% though, so not likely.

OTOH the most probable pump taken from random would be 0.004329 US gal * 20 --> 0.0866 gal. Or, iow, about what the 0.1 gal pump readout would indicate.

Of course, like any probability, what a particular realization will be is totally dependent upon the actual pump used but the (sizable) sampling of operating pumps taken during routine weights and measures compliance checks shows that in general they work pretty well with a few that have issues.

An interesting sidelight on that was the summary table of percentage failures (exceeding the 6 cu in threshold) by pump manufacturer. There were 4 with 100% compliance, another for in the high 80-90% range, another 4/5 in the low 80% and then one laggard at 73%. I'd not have guessed there were so many manufacturers but it appears there's a price/performance element there as is so often the case...

(The manufacturers were anonymous so no way to use the data to go find a station with one of the compliant pumps, unfortunately :) ). It did note that W&M compliance checks could be much more effective at a given cost/manpower level if used stratified sampling by vendor...

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Actually, the extrema limits in the sample are much worse: -50 and +146 in the 5 gal test collection. I simply computed the table for typical exceedance limits; didn't include the endmost points there. The 0.999 point, for example, is the int(20036*0.999)-th observation or 20015 so there were 21 more observations above that value. The last two, however, were really, really outliers that skew things quite a lot. The last five observations were

It's quite an oddity that there were 3 observations at 127; illustrating again that "random data aren't" or more correctly that one can always find patterns visually even in random data.

Of course, if the pump is putting out 5.6 gal/5 gal reading, you'll only have "pumped" 20*5/5.6 --> ~17.8 gal when the tank is full already... :)

Hmmm...I had noticed the outlier on the positive end and looked at it; wonder what the LH tail looks like now...

Pretty similar pattern, just not as extreme. There are two outliers separated quite a bit from the bulk of the rest of the observations, but they're only 10 cu in differential to nearest whereas there's 70+ on the other end.

Interesting that from a customer viewpoint you're just about as likely to get more than you're paying for as under and on the extreme ends by a lot more than by what you get shorted.

I didn't try to find what Canadian limits are -- I presume they must be somewhat more stringent in order to match, more or less, the size of the measurement interval? You know?

For the mathematically and engineering challenged, that is < 3.325 fluid ounces - or less than half a cup - or 3.25 in 640 - or an error of less than 0.5%

I do not know the requirements or test results today, but I DO know back when we had mechanical meters our pumps (at the stations where I worked)were never out by more than a couple oz in the 5 gallon calibration, and the new electronic metering pumps (deployed when we switched from imperial Gallons to Liters) were "significantly more accurate" in metering. The accuracy changed a bit with delivery speed on the mechanical pumps - can't remember if the change was that the read higher or lower with reduced pump speed, but the variance was quite low. The electronic meters were supposedly less susceptible to volume arrors based on fuel velocity.

The last years of my automotive career were not involved with gasoline retailing in any way - from a short time after the switch to Metric.

I haven't noticed if it is on all pumps but at least some of the ones I've used have a sticker reinforcing that the gasoline is dispensed by volume, not energy content. I've had a couple of times when I've filled a motorcycle tank with cool gasoline from underground storage tanks on a hot summer day and suddenly found I'd gotten more volume than I'd paid for -- and more volume than the tank would hold.

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As noted, being they're dispensing quantities measured in units that are roughly one-fourth the size one would suppose tolerances would be adjusted similarly. The 0.5% error with the US compliance standard would be almost 2% which seems as would be excessive in comparison to what a governing consumer-protection function would deem adequate.

Nothing came up in the searches I did, but I didn't look for non-US data, either, so being in US not surprising what I found was what I found... :)

Anyways, I think we can put the subject to rest... :)

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.

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

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

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.

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.

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 think my consistent mileage measurements support my conclusions.

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

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 keep saying this without indicating what you mean.

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

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.

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

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.

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...

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

dpb posted for all of us...

Where does one find common sense in da govt?

Yeah, as @patsajak noted (and especially in uber-liberal states like Oregon), politicians have learned that it's more fun to over-control people and tell them how to live than it is to fix the potholes

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?