I love this site. I used to use Personal Vehicle Tracker and loved it but this is better! I would like to see in the future the ability to track your maintenance and other misc stuff (car washes, etc). SMS/EMail/Twitter reminders when your 5,000 miles is up and time to get another oil change would be handy too!
If you've got the time (I'm sure you're swamped with coding...) you may want to do some googling on hypermilers. You might be able to get some sponsorship (or something along the lines) from the Wayne Gerdes or someone.
Also, along the lines of maintenance, it would be cool if there was a way to compare the results that maintenance has on a cars performance. For example, myself and 9 other people have the same car. I use synthetic oil, they use regular oil. which one gets better gas. Or I change my oil every 3k miles vs. 5, etc. I guess this would require you to set official maintenance terms that people can choose from, but it would definitely help.
Otherwise, cool idea, I'm gonna tell my friends and see which one of us can get the best gas mileage vs. what they say we should get.