Mileage per gallon (mpg) represents how efficiently your car uses fuel. For people who do not have a car that automatically calculates the mileage per gallon, you can calculate your mileage per gallon by hand. Comparing your car's mileage per gallon over time can help you notice when your car needs repairs. For example, according to the Department of Energy, replacing a bad oxygen sensor can boost your mileage up to 40 percent.
Fill your car's tank with gas.
Reset your trip odometer. If your car does not have a trip odometer, write down the mileage when you fill your tank.
Drive the car until you need to refuel. When you refuel, note the amount of gas you had to put in the car and the trip odometer reading. If you do not have a trip odometer, subtract the previous odometer reading from the final odometer reading to find out how far you drove. For example, if the initial reading was 28,500 and the final reading is 28,820, you would subtract 28,500 from 28,820 to get 320 miles driven.
Divide the number of miles driven by the number of gallons used to find your car's miles per gallon. Continuing the example, if you drove 320 miles and used 14 gallons, you would divide 320 by 14 to get 22.85 miles per gallon.
