Yes I just noticed that I had let Excel label the mpg to 2 decimal places - DUH. I'll fix that.

Obviously you need an accurate measure of the miles driven and the gallons of fuel consumed. The primary source of error in hand calculated mileage is that each time you fill up you must fill the tank to precisely the same level to get very accurate mpg calculations.

Note: you don't have to completely fill the tank; but you must fill it to the same level as you did the last time. The easy way to do this is to fill your tank using the same pump, set to the same flow rate and let the pump cut off automatically. This reduces the human 'topping it up' variable.

The more gals per fill up the more accurate the mpg. A .1 gal difference on 10 gal produces a roughly .25 mpg error assuming an average of 25 mpg. But at nearly a full tank (e.g. 20 gals), the same .1 gal error only produces a .125 mpg error. BTW .1 gal is the roughly the volume of can of Coke or a single click of most diesel pump handles. So I don't get too fussed about a few tenths of mpg on a 'per tankful calculation.

But the real beauty of the fuel log mpg calc. and graph, is not in the mpg diamonds; its in the mpglong line. The lifetime mpg actually gets more accurate with each fill up. Because over longer distances, the 'fill up' errors reduce to the difference between only the very first and very last fill up. IOW they become negligible

Funf Dreisig