When I review a car and drive it a reasonable distance I fill it at the start and note the mileage, then note the mileages each time I re-brim it, then calculate the real mpg. I do a regular run which is +/- 300 miles, so take 300 miles as the median and if a car records more or less I adjust the mileage figure by that. Of course drivers contributing to Real mpg may do it accurately or may simply read off the dash meter, but Real mpg is an average of an average of an average of all accuracies of measurement and all types of use so becomes a median and is therefore the most relevant figure.