Are your mpg figures accurate?

I went to fill an empty five gallon Jerry can and it took only 4.6 gallons
as indicated on the pump.

This is not uncommon as filling stations will pay a hefty fine for pumps
that over-read.

Thus, any mpg calculation I might make using this particular pump will
result in 8.5% better than true mileage.

Besides using a calibrated fuel measure, temperature is a small, but
measurable factor also. How many of you take that into consideration?

Then there is the calibrated odometer. If it over-registers, you mileage
deviates higher than true.

There are so many other factors including density altitude, winds, road
conditions, tire pressure, just to mention a few, it is futile to get any
meaningful measurements.

Besides, the factories pump up the tires to overpressure levels to boost
their figures not to mention other tricks like leaning out the mixture,
stripping out the spare and jack and even adding streamlining to massage
the advertising specs.