If you're looking to enter fuel purchases and mileage each time, then you can easily have a "rolling cumulative" average fuel consumption.
Just add up all the fuel added (use the SUM function) in gallons, and divide into the difference between latest mileage and "start" mileage.
I have a simple spreadsheet on my iPaq that does this very thing (in Excel).
There are blank lines for future entries, with an "IF" clause that leaves the average consumption column blank if there is no entry in the "fuel loaded" column for that line.