How to measure your gas mileage
- Fill the tank completely and reset the trip meter (or write down the odometer).
- Drive normally until you need fuel again. A full tank gives the most accurate number.
- Fill up again at the same pump if you can, and note how many gallons it took.
- Enter the miles and gallons above. The gallons from the second fill are exactly what you burned.
MPG formula
L/100 km = 235.215 ÷ MPG
Cost per mile = price per gallon ÷ MPG
Worked example
You drove 350 miles and it took 12.5 gallons to refill. 350 ÷ 12.5 = 28 mpg, which is 8.4 L/100 km. At $3.20 a gallon, every mile costs about 11.4 cents in fuel, or $114 per 1,000 miles.
Why your MPG is lower than the sticker
- Short trips and cold engines. An engine uses much more fuel in the first few minutes.
- Highway speed. Above about 50 mph, fuel use climbs quickly with speed because of air drag.
- Low tire pressure. Under-inflated tires add rolling resistance. Check pressure monthly.
- Extra weight and roof racks. A roof box can cut highway economy noticeably.
- Idling and hard acceleration. Smooth driving is the cheapest upgrade there is.
MPG to L/100 km
| MPG (US) | L/100 km | Fuel cost per mile at $3.20 |
|---|---|---|
| 15 | 15.7 | $0.21 |
| 20 | 11.8 | $0.16 |
| 25 | 9.4 | $0.13 |
| 30 | 7.8 | $0.11 |
| 40 | 5.9 | $0.08 |
| 50 | 4.7 | $0.06 |
Planning a drive? Use the fuel cost calculator. Comparing with an electric car? Try the EV charging cost calculator.
Frequently asked questions
How do I calculate my gas mileage?
Divide the miles you drove by the gallons it took to refill the tank. For example, 350 miles on 12.5 gallons is 28 miles per gallon.
What is good gas mileage?
For a gasoline car in the US, 30 mpg or more is good, and many hybrids reach 45 to 55 mpg. Pickups and large SUVs typically get 15 to 25 mpg.
How do I convert MPG to liters per 100 km?
Divide 235.215 by the MPG. For example, 28 mpg equals 8.4 liters per 100 km.
Is the MPG on my dashboard accurate?
Dashboard readings are often a few percent optimistic. Measuring miles and gallons at the pump over several fill-ups gives a more accurate figure.
How much does it cost me per mile to drive?
Divide the price per gallon by your MPG. At $3.20 a gallon and 28 mpg, fuel costs about 11.4 cents per mile.
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Last updated October 2026.