well i also log mg/stroke rpm and various other things.
I've started rolling sum of litres per sample rate, and km per sample rate, then dividing one by the other. this gets over the issue of the 0 and low fuel usage, but assumes speed and l/h are constant over about 1.25s.
this gives me about 62mpg (slightly better than when i ignore the funny numbers)
as you say, i should just average it out per tank, but my inner geek is crying out for attention
what i was hoping for was a way to write a bit of VBA to display my instant and average on my netbook screen and using it as a economy gauge as i drive along.
i guess the next stage is working out the language of my vag-com cable to pull the numbers out directly into excel.