How to measure your MPG
Fill the tank until the pump clicks off and note the odometer reading, or reset a trip odometer. Drive normally. At your next fill-up, fill until the pump clicks off again and note the gallons on the receipt and the new odometer reading. Miles driven divided by gallons added is your miles per gallon for that tank.
Use the same pump-shutoff method each time and don't top off. A single tank can be off by a few percent because pumps stop at slightly different levels, so track three or more fill-ups and divide total miles by total gallons for a steadier figure.
MPG vs liters per 100 km
MPG measures distance per unit of fuel, so higher is better. Liters per 100 km, used in Canada, Europe and most of the world, measures fuel per unit of distance, so lower is better. To convert, divide 235.21 by the number: 26.39 MPG is 8.91 L/100 km, and 8.91 L/100 km is 26.39 MPG.
Fuel-per-distance is the more honest way to compare cars, because it scales directly with what you burn. Going from 15 to 20 MPG saves 1.67 gallons every 100 miles, while going from 40 to 50 MPG saves only 0.5 gallons. The table under the result shows both scales side by side.
Why your MPG is lower than the sticker
EPA ratings come from standardized lab tests, and real-world results vary with how and where you drive. Short trips with a cold engine, stop-and-go traffic, high speeds, heavy loads, roof racks, under-inflated tires, extreme temperatures and running the air conditioning all cut fuel economy.
Driving style matters most. FuelEconomy.gov, run by the Department of Energy and EPA, says aggressive driving such as speeding and rapid acceleration and braking can lower gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic. A sudden drop in your usual MPG can also be an early sign of a maintenance problem worth checking.
The formulas
MPG = M ÷ GM = odometer now − odometer at last fill-upL/100 km = 235.215 ÷ MPG- M = miles driven since the last fill-up
- G = gallons needed to refill the tank
- 235.215 = 100 × 3.785411784 liters per US gallon ÷ 1.609344 km per mile
Example
- Your odometer read 45,210.0 at a fill-up and 45,566.2 at the next one, when the tank took 13.5 gallons.
- Miles driven = 45,566.2 − 45,210.0 = 356.2 miles.
- MPG = 356.2 ÷ 13.5 = 26.39, or about 26.4 MPG.
- L/100 km = 235.215 ÷ 26.39 = 8.91 L/100 km.
- At an example price of $3.50 a gallon, that tank cost $47.25 and fuel cost about 13.3 cents a mile.
Frequently asked questions
How do I calculate gas mileage?
Divide the miles you drove by the gallons it took to refill the tank. 356.2 miles on 13.5 gallons is 26.4 MPG.
How do I convert MPG to L/100 km?
Divide 235.215 by the MPG. 30 MPG is 7.84 L/100 km; 25 MPG is 9.41 L/100 km. The same division converts L/100 km back to MPG.
Should I use the car's trip computer?
It's convenient, but onboard MPG displays are estimates and can read a little high or low. Checking against the pump receipt now and then tells you how far off yours is.
What is good gas mileage?
It depends on the vehicle. The fairest comparison is with the EPA combined rating for your model on FuelEconomy.gov: if you're well below it over several tanks, look at driving habits, tire pressure and maintenance.
Does using a partial fill-up work?
Only if you fill to the same point each time. If you put in a set dollar amount instead of filling the tank, the gallons won't match the miles driven. Fill to the pump click-off at both ends.
Sources
Last reviewed for 2026. How we calculate.