How to measure your real MPG
- Fill the tank completely and reset your trip meter.
- Drive normally until you need fuel again, ideally at least half a tank.
- Fill up completely again and note the amount of fuel added from the pump receipt and the distance on the trip meter.
- Enter both numbers above. The fuel you just added is exactly the fuel you used over that distance.
Repeat over two or three tanks for a reliable average. A single tank can be skewed by a long highway trip, a cold week or stopping the pump at a slightly different point.
The MPG formula
MPG = miles driven ÷ gallons used
L/100 km = liters used ÷ km driven × 100
The two systems run in opposite directions: a higher mpg is better, but a lower L/100 km is better. To convert, divide 235.21 by either figure: 30 mpg = 235.21 ÷ 30 = 7.84 L/100 km.
Worked example: you drove 350 miles and the tank took 11.5 gallons to refill. 350 ÷ 11.5 = 30.4 mpg, which is 7.73 L/100 km. At $3.50 a gallon the fill-up cost $40.25, or 11.5 cents per mile. If you drive 1,000 miles a month, fuel costs about $115 a month and $1,380 a year.
Fuel economy conversion table
| US mpg | L/100 km | km/L | UK mpg |
|---|---|---|---|
| 15 | 15.7 | 6.4 | 18.0 |
| 20 | 11.8 | 8.5 | 24.0 |
| 25 | 9.4 | 10.6 | 30.0 |
| 30 | 7.8 | 12.8 | 36.0 |
| 40 | 5.9 | 17.0 | 48.0 |
| 50 | 4.7 | 21.3 | 60.0 |
UK mpg figures are about 20% higher than US mpg for the same car, because an imperial gallon (4.546 L) is larger than a US gallon (3.785 L).
Why your MPG is lower than the sticker
- Short trips and cold starts. An engine uses much more fuel until it reaches operating temperature.
- Winter fuel and weather. Cold air, winter-blend fuel and heater use can lower economy by 10–20%.
- Under-inflated tires, roof racks and extra weight all add resistance.
- Driving style. Quick acceleration, hard braking and high highway speeds cost more than anything else you control.
Planning a drive? Use your measured figure in the fuel cost calculator for an accurate trip estimate.
Frequently asked questions
How do I calculate my car's MPG?
Fill the tank, reset the trip meter, drive, then fill up again. Divide the miles on the trip meter by the gallons it took to refill. For example, 350 miles ÷ 11.5 gallons = 30.4 mpg.
How do I convert mpg to L/100 km?
Divide 235.21 by the mpg figure. 25 mpg is 235.21 ÷ 25 = 9.4 L/100 km. The same formula works in reverse: 235.21 ÷ L/100 km gives US mpg. For UK mpg, use 282.48 instead.
Is the MPG on my dashboard accurate?
It's usually close, but many trip computers read a few percent high. The fill-up method is the most reliable way to know your real fuel economy.
What is good gas mileage?
For a modern gas car, 30 mpg (7.8 L/100 km) or better is good for a midsize sedan, and 25 mpg (9.4 L/100 km) is good for a midsize SUV. Hybrids commonly reach 45–55 mpg (4.3–5.2 L/100 km).
Results depend on the numbers you enter. Fill-ups that stop at slightly different points can make a single tank's result less accurate.