This app can track the MPG for multiple cars. Input information about each car you would like to track, and then information about each time you fill up the vehicle with gas. You can update information you entered previously. It's not super pretty, but nicer navigation will be included in the next update. The next update will also include changes that allow incomplete entries and cars without a nickname.
NOTE: I found that the MPG for the first entry is not calculated properly. Everything after that entry does. I will be updating this ASAP.