I have just posted this in a posting on the forum but thought it might be useful to stick it here too:

A year or so ago, a friend introduced me to http://www.fuelly.com which is free to join. It tracks your mpg to put it simple. Basically when you fill up, on your receipt will be how much you spent, price per litre and how many litres you filled up with. Each time you fill up, reset your trip meter and go to the fuelly website with how much you filled up with etc and it works out your fuel