Would you guys actually use a web app to track your gas mileage? Seems like a lot of worrying over a few bucks. Heck, on a good day I get 5 mpg, I'm better off calculating distance to the next gas station instead of miles per gallon.
Fuelly is a handy little web application that's easy to use from your smartphone, or your computer. It tracks your gas purchases and mileage over time and calculates miles per gallon overall and per tank.