Calculate the miles per gallon (MPG) from the odometer readings and the amount of fuel you add each time. Works for multiple vehicles in the same account. Works for old readings, i.e. you can add readings from the past and specify the dates the readings are from.
Each time you add fuel to the vehicle, fill up to the max and record the odometer, fuel volume, cost, and date. Then enter the readings into the app each time to calculate your MPG over time.