Fill the fuel tank completely
Reset the trip odometer to zero
Drive normally until you need to refuel
Refill the tank completely
Record the number of gallons or liters added
Record the miles or kilometers driven on the trip odometer
Divide miles driven by gallons used to get miles per gallon
Divide kilometers driven by liters used to get kilometers per liter
Multiply kilometers per liter by 2.352 to get miles per gallon
Divide 100 by liters per 100 kilometers if using metric fuel consumption
Repeat over multiple tanks for a more accurate average