This gas mileage calculator computes MPG from distance and fuel used: MPG = miles / gallons.
Example: 300 miles on 10 gallons is 30.00 MPG. Keep units consistent.
Turn efficiency into trip spend with the fuel cost calculator. For reimbursement style mileage amounts, see the mileage calculator.
Converting MPG to L/100 km and back
One U.S. gallon is 3.785411784 litres and one mile is 1.609344 km. That gives an exact constant:
L/100 km = 235.215 ÷ MPG, and MPG = 235.215 ÷ L/100 km.
L/100 km is an inverse scale, so lower is better. Going from 20 to 25 MPG saves more fuel than going from 40 to 50 MPG. That is easier to see in litres (11.8 → 9.4 vs. 5.9 → 4.7 L/100 km). If you use UK gallons, which are larger, the constant is about 282.5. Do not mix the two.
Gas mileage calculator example: one tank, three ways
You fill to full, drive 412.6 miles, and the next fill to full takes 13.8 gallons.
- MPG: 412.6 ÷ 13.8 = 29.9 MPG
- L/100 km: 235.215 ÷ 29.9 = 7.87 L/100 km
- Against the EPA rating: if the label says 32 combined, you are at 29.9 ÷ 32 = 93% of the sticker, which is within normal variation.
Why your MPG differs from the EPA label
EPA ratings come from standardized lab tests so that vehicles can be compared fairly. The combined figure weights city driving 55% and highway 45%. The 2026 Fuel Economy Guide lists factors that lower real-world mileage:
- Aggressive driving, stop-and-go traffic and excessive idling
- Cold weather, which hurts most on short trips
- Heavy loads, roof cargo and air-conditioner use
- Under-inflated tires, an engine needing service, hilly terrain and snow tires
- A new engine that is not yet broken in, typically 3,000–5,000 miles
How much habits change your result
fueleconomy.gov estimates that aggressive driving (speeding, rapid acceleration and hard braking) can lower gas mileage by roughly 15%–30% at highway speeds and 10%–40% in stop-and-go traffic. In an Oak Ridge National Laboratory study of 74 vehicles, most were most efficient around 40 mph. Economy fell sharply above 50 mph, with an average drop of 36% from 50 to 80 mph.
To see your own effect, log three full-to-full tanks at your usual pace and three at calmer speeds. Then compare the weighted averages, using total miles ÷ total gallons.
Getting a clean full-to-full reading
Fill until the pump clicks off the first time, at the same station and pump if you can, and do not top off. Reset the trip meter at that fill. Odometer-based miles divided by pump gallons is what this gas mileage calculator expects. Averaging at least three tanks smooths out pump shut-off differences and weather. The 2026 Fuel Economy Guide notes that a single test cannot predict mileage for every driver, and neither can a single tank.
Hybrids and plug-ins
For a plug-in hybrid, pump-only MPG overstates efficiency, because part of the distance runs on grid electricity. Track electricity (kWh) separately. The EPA rates PHEVs in two modes, electric/blended and gasoline-only.
Keep a simple log in your phone or glovebox. Note the date, odometer reading, gallons added and the price, then enter each tank in the gas mileage calculator. After a few weeks the pattern shows which routes, seasons and habits cost you the most.
If a result looks far too high or low, check for a missed fill-up or a pump that stopped early. One bad entry can skew the average, so correct it before you draw any conclusions about the vehicle.
Finally, remember that tire pressure, fuel quality and weather all shift the numbers a little from tank to tank.
Last reviewed: October 2026