How about adding a module for tracking your cycling/running activities? I usually have my Blackberry at the end of a ride, so perfect time to log miles. I ride 3 different bicycles (road, mountain, & cyclocross), so it would be cool to have all the numbers collected into a single database. Only minor changes needed--instead of gallons you'd enter time (hours & minutes) and it would give you your average MPH. Already doing this--just added an extra vehicle and enter time in the gallons field in decimal, but could be better.
It's a good idea and of course the logical extension of saving fuel is eventually not driving and riding a bike or walking instead, but that said, there are several good existing services that do that already and have been for years. We were even inspired by them somewhat.
We Endure is a simple exercise tracking site for logging your swims, runs, and rides. Lately I've also started using RunKeeper, which works with iPhones and Android devices to track your workouts via GPS.