Gas Mileage Calculator
Work out your car's real gas mileage (MPG) from the miles driven and gallons used between two fill-ups, or show it in liters per 100 km.
Last updated: October 2026
Fuel economy
30 mpg
Formula below · 2 sources (fueleconomy.gov, NIST) · Updated Oct 2026
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About this calculator
Gas mileage, or fuel economy, is how far your vehicle goes on one gallon of fuel. Measuring it yourself over a full tank gives a more honest number than the dashboard estimate.
The formula
MPG = miles driven ÷ gallons used. 300 miles on 10 gallons is 30 MPG. To compare with metric figures, liters per 100 km = 235.215 ÷ MPG, so 30 MPG is 7.84 L/100 km; lower is better in that unit.
How to measure it
Fill the tank completely and reset the trip meter (or note the odometer). Drive normally. At the next fill-up, fill completely again and note the gallons pumped and the miles driven since the last fill. Divide miles by gallons. Averaging several tanks smooths out pump shut-off differences.
Why your number differs from the window sticker
FuelEconomy.gov lists many factors that lower real-world mileage. Aggressive driving (speeding, rapid acceleration and hard braking) can cut gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic. Mileage usually drops quickly above 50 mph, and a large roof-top cargo box can cost 10% to 25% at interstate speeds. Cold weather, short trips, idling and under-inflated tires also hurt.
| MPG | L/100 km | Gallons per 100 miles |
|---|---|---|
| 15 | 15.68 | 6.67 |
| 20 | 11.76 | 5 |
| 25 | 9.41 | 4 |
| 30 | 7.84 | 3.33 |
| 35 | 6.72 | 2.86 |
| 40 | 5.88 | 2.5 |
| 50 | 4.7 | 2 |
How to use
Enter the miles driven since your last full tank and the gallons it took to refill. The defaults, 300 miles on 10 gallons, give 30 MPG, or 7.84 L/100 km if you switch the result. Use the odometer difference rather than the range shown on the dashboard, and fill to the first automatic shut-off both times for consistent results.
Frequently asked questions
How do I calculate my gas mileage?
Fill the tank, note the odometer, drive, then refill and divide the miles driven by the gallons it took to refill. 320 miles on 11.5 gallons is 27.8 MPG.
How do I convert MPG to liters per 100 km?
Divide 235.215 by the MPG. 25 MPG is 9.41 L/100 km and 40 MPG is 5.88 L/100 km. The same division converts L/100 km back to MPG.
Why is my MPG lower than the EPA rating?
Speed, aggressive acceleration and braking, cold weather, short trips, idling, roof racks and low tire pressure all lower real-world mileage. FuelEconomy.gov estimates aggressive driving alone can cost 15–30% at highway speeds.
Does the dashboard MPG readout match this?
Often within a few percent, but trip computers estimate fuel use from injector data and can read high. A fill-to-fill calculation over several tanks is the most reliable check.
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Sources & references
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