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

Darn right you did.

Bull crap

Bull crap

Bull crap

So? Both can and do.

WEhich any grade 5 student could do (at least when I was in grade 5)

You open your eyes, and get over it. What car do you have with no tenths on either the main or trip odo???? Use your head and write down the start mileage, accurate top the tenth on the main ODO if you happen to have the rasre car with no tenths on the tripmeter.

You would lose your bet

If yopu are an engineer too stupid to find a way around it, I suppose so.

And a deaf, dunb and blind messanger. A pedantic child - - -

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,

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:

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.

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...

Oh my, Clare and his new buddy aren't getting along well at all.

Wrong. If a pump has an accuracy of 1%, then it has an accuracy of

Per above, Mad Roger is right on that one. Do you really think a pump that reads out in liters is going to be more accurate than the same pump that reads out in gallons? It's just a math conversion going to the display.

I don't recall Mad Roger claiming to be an engineer. I think he said he was a "scientist" with vague references to biology or life sciences.

PS: Why can't you trim posts?

I noted the same thing in his other thread about wheel sizes. Right from the start Mad Roger made the wrong assumption that torque at the wheel translated directly into fuel usage. He was conflating torque with work/energy. So, I carefully explained it, gave him examples of lifting a 100lb rock up 2ft using a 4 ft lever or a 10 ft lever. While the torque is different, the work/energy used is exactly the same. Gave him another example of a bicycle going uphill with different gear ratios. Did he acknowledge that, discuss it? No, just like with your example, instead he obfuscated by then bringing up wind resistance and continued to troll on. He also claimed to be a "scientist", then he says that DPB's post about fuel pump accuracy was confusing because he used cubic inches and gallons?

We don't agree on a lot things, but we sure agree on this!

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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... :)

I don't see any reason that the accuracy of pumps that measure in liters would be any different than the accuracy of pumps measuring in gallons. The actual quantities dispensed are the same, you'd think the pumps probably use very similar measuring technology, maybe even exactly the same pumps, just that one reads out in gallons, the other liters. Also, since the obvious requirement standards for accuracy is so that the consumer doesn't get screwed and the transaction is far, it would seem that the standards would be set similarly. If the US standard is to be off by no more than 6 cubic inches in 5 gallons, then I'd expect the EU standard would be similar, eg 100 ml in 20 liters. That puts in at ~ 0.5% accuracy which is reasonable for a consumer transaction. I doubt milk, olive oil or anything else is required to be any more accurate than that.

On 07/23/2017 2:18 PM, trader_4 wrote: ...

That's actually probably the case; somewhere along the way I ended up with a number from the NIST data that came out pretty close to 0.1 gal and made a connection that's likely not there in reality. Now I don't recall just where the number came from, even--and, it might've been a misstep along the way, besides. :)

I do think it somewhat amusing that NIST is still using the '6 cu in/5 gal' paradigm given they're the "standards" folks. :) I didn't try to research the history but I'm sure this is a historical artifact that probably goes back to the '20s or '30s and it's just too ingrained to change.