I've downloaded "TankBuch" to try it out (it is the only free one out there). It's okay, but since it was written outside of the U.S., some of the terminology takes some getting used to. The graph is difficult to interpret, IMO. It's rating on the AppStore is 3 stars from 30 reviews.
What about
Appigo's AccuFuel? It has 4 stars from 106 reviews. I've lurked around in their support forum online and they seem to be very responsive to everyone's requests and suggestions. Looking at the screen shots, it also looks more "polished" than the other apps.
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