I've started keeping an OO.o spreadsheet of my fueling activity.

It contains the date, miles on the trip meter, gallons purchased, the reported MPG for the tank from the computer, and the caluclated MPG from the miles/gallons. I then look at the error rate and I can figure out lifetime MPG as well as the equivalent number from the reported MPG (take the reported MPG, multiply by the miles driven, add all the tanks together then divide by the sum of the miles driven). It sounds complicated, but I really only have to enter in the miles, gallons and computer MPG and the rest is calculated automatically. The only problem I have is I am not fluent enough in OO.o yet to make it do any nifty charts or anything. Maybe if I get it a little more polished I'll offer it as a template if anyone wants to use it.