Yeah, as we get more data, we're definitely planning to show more averages across the system (MPG for all Honda Civic hatchbacks with a 4 cyl engine, then all Honda Civic Hatchbacks, then all Honda Civics of any body style, then for all Hondas, etc). At the moment, the site is barely a week old and we're still trying to get all the error-checking in place to weed out the bad outlier data (I'd say 5% of data looks like bad text entry messing up calculations). So definitely in the next few months when we have much more data, we'll be showing off much more of it.
You can see total users, total cars, total fill ups, and total miles driven on the front page of Fuelly if you log out and look below the screencast video. We might add that to the default dashboard for logged in users as well, to give them a sense of how big the community has gotten in just a week.