I like it! I think it would be cool to set up the occasional competition between different teams for a month or two. Turns an individual 'sport' into a team sport
My suggestions, which are probably already on your list:
-On each team page, show each member and their stats, and a picture for their car, if they have one.
-Make it so the 'leader' of the team has to approve each member, or some system to prevent people with Honda's joining team Toyota, or something like that.
-Make an error message show up when you try to join a team that you're already a part of.
How are the stats calculated? By total miles/total gallons, or by ΣMPG/# of team members?