GasUpLogger is a nice little program that lets you follow and record your vehicles' fuel consumption. All you have to do is enter the mileage/kilometer reading and the amount of fuel.
The program is very well constructed, and does not calculate averages unless one fills up the tank. Makes sense.

It records data in files that can be exported in CSV format. Each vehicle will have its own file. Obviously, you can get the same information with an Excel spreadsheet, but it will never be this simple