[gnhuk]

Relying on a fuel computer would make the whole process meaningless. The only way to realisticly collect the data is to record the mileage via the trip meter and write down the amount of fuel taken to brim the tank as Viper_7 states. Okay the tripmeter might not be totally accurate but, assuming you have the right tyre size, the inaccuracy will be as constant - if that is the case it would be possible to apply a correction factor to the results. Besides everyone can do it this way, by restricting it to only those who have the computer you are excluding a large number of people so your results will be unrepresentative.

I record the mileage, fuel amount and cost in a spread sheet so I can spot trends (and possible problems) as they occur. It is not in my interest to tell porkies.