Petrol Patrol
Petrol Patrol is an Android app designed to help you keep track of all those numbers you get when you top off your car.
It includes a simple interface because, let's face it, who wants to do a ton of fancy, complicated stuff when you just want to leave the gas station? That's not to say that it can't do neat stuff, however, as planned features include visualizations of your fill-ups, including average MPG, and graphs of price, MPG, and distance.
It is not on the Play Store yet as there is still some features and polish to put into it, and a neat-ification of the UI. If you want you can always compile from the source code and start using it now if it intrigues you!