http://www.fuelly.com/

I have been using this website for a while to track the fuel mileage of my PT Cruiser GT. It is real sweet, you add in each fillup (you can track by odometer reading or trip mileage) and it calculates your MPG, shows you graphs for your fuel mileage trends, and does all kinds of other cool stuff. You can add in your fuel mileage from your mobile phone while on the road and even compare your mileage with other owners of the same car. You can even look at graphs showing other owner's fuel mileage trends.

Awesome legit website, never recieved any spam from them. I noticed there are only 3 Crossfire Coupes on there other than mine!

If you sign up feel free to add me as a friend. My name on Fuelly is "scrible"