I’ve been using FuelFrog since May to track my gas mileage. Today I imported the four entries on FuelFrog to Fuelly, a site created by Matt Haughey (of Metafilter) and Paul Bausch (of OnFocus). It has more graphs and numbers (it congratulates you on your best tank of gas) and adds a social element to your gas mileage. You can see what and how your friends drive.
Here are my Maxima’s stats.