Miles per gallon can be calculated by using the odometer method when filling the vehicle's tank with gas. This method uses four easy steps to calculate miles per gallon.
When utilizing the odometer method, fill the vehicle's gas tank completely, and write down the odometer reading. When the tank is empty and needs refilling, write down the new odometer reading and the number of gallons it takes to fill the tank completely. Subtract the previous odometer reading from the new one to calculate the distance driven. Divide the number of miles driven by the number of gallons it took to fill the tank. The result is the vehicle's miles per gallon.