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

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

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

Why do errors compound in your view?

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

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

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

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

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