i've got a spreadsheet for my fuel usage for both cars

along the top it says:

date - mileage - distance covered - fuel used (litres) - fuel used (gallons) - cost per litre - litres/mile - total cost - mpg

the date is obvious, when you fill up
the mileage is the total mileage on the odometer
the program then works out the miles covered since the last fill up
then i input how many litres of fuel i put in
the program then coverts this to gallons
then i input the cost per litre of fuel
and by using maths, the program then spits out how much it's cost me per mile and how much the mpg was
the last 3 fill ups on the mondeo worked out to be 16.71 mpg, 22.68mpg and 20.33 mpg
only started to do the fiesta this past week and that was 21.07 mpg (but it's all stop start and 1 mile there and 1 mile back type of stuff)

over the course of a few months you should be able to see a decent pattern forming, and if you start to add in where you filled up and what you put into it, it may also show you which fuel is best for economy too