Fuel Economy Calculator
Calculate your measured fuel economy in US mpg, L/100 km, imperial mpg and km/L, plus fuel cost per mile and per kilometer.
Updated Formula covered by automated tests
The number on a window sticker is a laboratory estimate; the number that matters is what your own car does on your own roads. Record the odometer at a fill-up, drive normally, fill the tank again and note how much fuel went in. This calculator turns that distance and fuel into miles per US gallon, liters per 100 kilometers, miles per imperial gallon and kilometers per liter, and, if you add the pump price, into cost per mile and per kilometer.
Showing all four figures side by side is handy when you read a review written in another country. A British magazine quoting 45 mpg is using larger imperial gallons, so the same car would be rated about 37.5 mpg in the United States, while European and Canadian sources quote liters per 100 km, where a smaller number is better. You can enter your trip either in miles and US gallons or in kilometers and liters.
The formula
- miles, kilometers
- Distance driven between the two fill-ups; 1 mile = 1.609344 km exactly
- US gallons, liters
- Fuel added at the second fill-up; 1 US gallon = 3.785411784 L exactly
- imperial gallon
- The UK gallon, defined as 4.54609 L exactly, about 20% larger than the US gallon
- 235.214583
- 100 × 3.785411784 ÷ 1.609344, the constant that links US mpg and L/100 km
Miles per gallon measures distance per unit of fuel, while liters per 100 km measures fuel per unit of distance, so the two are reciprocals. Converting one into the other is a division, not a multiplication, which is why 20 mpg is 11.76 L/100 km and 40 mpg is 5.88 L/100 km: doubling mpg halves fuel use. The constants come from the exact international definitions of the mile, the US gallon and the imperial gallon.
The reciprocal relationship has a practical consequence that researchers call the MPG illusion. Raising a car from 10 to 20 mpg saves 5 gallons every 100 miles, but raising another from 30 to 60 mpg saves only about 1.7 gallons over the same distance. Fuel per distance (L/100 km, or the gallons per 100 miles figure printed on newer US labels) shows real savings directly.
Worked example
Rosa fills her sedan, drives 312.4 miles of mixed commuting, and the second fill-up takes 11.2 US gallons at $3.49 a gallon.
- 1 Miles per gallon: 312.4 ÷ 11.2 = 27.89 mpg (US).
- 2 Liters per 100 km: 235.214583 ÷ 27.89 = 8.43 L/100 km.
- 3 Imperial mpg: 27.89 × 1.20095 = 33.5 mpg (UK).
- 4 Fuel cost: 11.2 × 3.49 = $39.09, so cost per mile is 39.09 ÷ 312.4 = $0.125.
Result: Rosa’s car averaged 27.9 mpg (8.43 L/100 km), and fuel cost her about 12.5 cents per mile on this tank.
Before you rely on the result
- One tank of data reflects that tank’s mix of driving, weather and traffic; use several fill-ups before drawing conclusions about the car.
- Cost per mile covers fuel only, not maintenance, depreciation, insurance or tolls, which usually exceed fuel in the total cost of driving.
- For plug-in hybrids the result ignores electricity drawn from the grid, so it overstates efficiency.
Frequently asked questions
Why is my real mpg lower than the EPA estimate?
The EPA rating comes from standardized lab cycles adjusted to reflect typical driving. Short trips with a cold engine, high highway speeds, aggressive acceleration, heavy air-conditioning use, cold weather, roof racks and underinflated tires all lower real-world results. Measuring over several tanks gives a fairer average than a single fill-up.
How accurate is the fill-up method?
It depends on filling to the same point both times. Let the pump click off automatically once and do not top off, ideally at the same pump. Over one tank an error of half a gallon is easy; averaging three or more tanks smooths it out. The trip computer in the dashboard is convenient but can be several percent optimistic.
Why do UK and US mpg figures for the same car differ?
They use different gallons. The imperial gallon is 4.54609 liters and the US gallon is 3.785411784 liters, so a UK mpg figure is about 1.2 times the US figure for the same fuel consumption. Divide a UK number by 1.20095 to compare it with a US one.
Is a lower L/100 km better?
Yes. Liters per 100 km is fuel consumed, so lower means more efficient, the opposite direction from mpg. A typical compact car might use 6 to 8 L/100 km, while a large pickup can use 12 or more.
Sources
- US DOE and EPA: fueleconomy.gov (fueleconomy.gov)
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement (nist.gov)