This is an amazing app I've been using for a full month now. It's called FillUp. It keeps track of fuel mileage, fuel cost, fuel cost per mile, etc. Then it compiles them into statistics over specific periods of time. There are plenty of apps that do similar things but what got me about this app was how it can keep track of everything even on partial fills. This little app is actually helping me use less gas because I'm paying attention to it.
https://play.google.com/store/apps/details?id=com.github.wdkapps.fillup
https://play.google.com/store/apps/details?id=com.github.wdkapps.fillup