The MPG Formula

Miles per gallon is the simplest measure of fuel efficiency. The formula is:

MPG = Miles Driven ÷ Gallons Used

To get an accurate reading, fill your tank completely, reset your trip odometer, drive normally until the tank is near empty, then fill up again. Record the number of miles on the odometer and the gallons pumped. Divide miles by gallons — that is your real-world MPG.

Example: You drove 320 miles and used 11.4 gallons of gas. MPG = 320 ÷ 11.4 = 28.1 MPG. This is your actual observed fuel economy — not the EPA estimate, which is tested in controlled conditions that rarely match real-world driving.

Why Your Real MPG Differs

EPA window sticker MPG ratings are tested in a laboratory under specific conditions — controlled temperature, no air conditioning, no cargo, smooth surfaces, predictable acceleration. Real-world driving differs dramatically.

Highway vs. city driving has the biggest impact. City driving with frequent stops and starts burns far more fuel per mile than steady highway cruising. A car rated 28 MPG combined might achieve 22 MPG in urban stop-and-go and 34 MPG on a long highway trip.