If being proper geeky don't forget to measure actual distance driven and not the variable rolling circumference calculated.
Cars over read as a matter of course of speed, that only gets worse as tyres wear down.
Even when done by brim to brim distances travelled are inaccurate unless you log actual real distances accumulated.
I used to have a sat available distance travelled based fuel economy tool on my older 1690T garmin sat nav, the closest distance tally was the mog!
