This will work for Miles/Gallons or Kilometres/Litres.

Calculate the mileage and cost of fuel based on odometer reading and a tank of fuel.

For this to work you need to write down the odometer reading the first time you fill up and the cost per gallon/litre of your fuel at this time. After driving for a while and it is time to fill up again take a second odometer reading and the amount of fuel needed to re-fill your vehicle's tank.

The Calculator

1. number of miles/kilometres on odometer at fill up
2. price of fuel per Gallon/Litre at this fill up
3. number of miles/kilometres on odometer at second fill up
4. amount of gas needed to fill up vehicle's tank the second time