Each time you fill your vehicle's fuel tank, just enter the odometer reading and the fuel amount. Mileage Keeper does the rest! See fuel economy for each refueling as well as your vehicle's overall average. It also calculates fuel cost per refueling and per vehicle. Best of all - it's free!
Tap the '+' in the upper-right of the app to add new vehicles. To add a refueling for a vehicle, first tap on the vehicle on the main page, then tap the '+' in the upper-right. Once you have entered two refuelings for a vehicle, it will show you your per-tank mileage on the 'refuelings' page, and it will show you the average mileage for each vehicle on the main Vehicles page.