Distance per fuel, or fuel per distance
Fuel figures come in two opposite shapes:
- Economy: distance traveled per unit of fuel. Miles per gallon (mpg) and kilometers per liter (km/L) work this way, and a bigger number is better. The United States quotes mpg with the US gallon; the United Kingdom quotes mpg with the imperial gallon, which is 20% larger, so the same car scores higher in UK mpg. Kilometers per liter is the unit of Japan’s official ratings and is used in some other markets too.
- Consumption: fuel used per fixed distance. Liters per 100 km (L/100 km) is the standard in Europe, Australia, Canada, China and many other countries, and a smaller number is better.
Converting between the two shapes is not a multiplication but a reciprocal: L/100 km = 100 ÷ km/L. That is why the working line for this pair says the units are not proportional instead of showing a factor. Between the two mpg units and km/L, which are all distance per fuel, the converter shows an ordinary factor (1 US mpg = 1.2009 UK mpg).
Fuel economy formulas
The constants come from the exact mile (1.609344 km), US gallon (3.785411784 L) and imperial gallon (4.54609 L), rounded here to nine or ten significant digits. Because the first two are divisions, the formula works the same way in both directions: mpg = 235.214583 ÷ L/100 km.
Why averaging mpg misleads
Suppose two cars each drive 100 miles, one at 20 mpg and one at 40 mpg. The average of the two numbers is 30 mpg, but the fuel tells another story: 5 gallons plus 2.5 gallons is 7.5 gallons for 200 miles, which is 26.67 mpg. A distance-per-fuel figure cannot be averaged directly because the thirstier car burns more of the fuel. In L/100 km, the same two cars are 11.76 and 5.88; their plain average, 8.82 L/100 km, matches the 26.67 mpg answer exactly, because consumption units add up over equal distances.
The same effect hides savings. The US fuel economy label notes that going from 10 to 15 mpg saves about 33 gallons per 1,000 miles, while 30 to 35 mpg saves only about 5, which is why the label also gives a fuel consumption rate in gallons per 100 miles.
Worked examples
- Divide the constant by the mpg: 235.214583 ÷ 30 = 7.8405 L/100 km.
- To US mpg: 50 ÷ 1.200949926 = 41.634 mpg (US).
- To L/100 km: 282.480936 ÷ 50 = 5.6496 L/100 km.
- US: 235.214583 ÷ 6 = 39.202 mpg.
- UK: 282.480936 ÷ 6 = 47.080 mpg.
- Divide 100 by the km/L figure: 100 ÷ 15 = 6.667 L/100 km.
US mpg conversion table
| Miles per US gallon (mpg (US)) | Liters per 100 kilometers (L/100 km) | Miles per imperial gallon (mpg (UK)) | Kilometers per liter (km/L) |
|---|---|---|---|
| 10 | 23.52 | 12.01 | 4.251 |
| 15 | 15.68 | 18.01 | 6.377 |
| 20 | 11.76 | 24.02 | 8.503 |
| 25 | 9.409 | 30.02 | 10.63 |
| 30 | 7.84 | 36.03 | 12.75 |
| 35 | 6.72 | 42.03 | 14.88 |
| 40 | 5.88 | 48.04 | 17.01 |
| 45 | 5.227 | 54.04 | 19.13 |
| 50 | 4.704 | 60.05 | 21.26 |
| 60 | 3.92 | 72.06 | 25.51 |
Read down the L/100 km column: the first 5 mpg of improvement saves almost 8 L every 100 km, while going from 45 to 50 mpg saves about half a liter.
Fuel figure mistakes
- Comparing US and UK mpg as if they matched. A 40 mpg rating from a British review is about 33 mpg in US terms. Check which gallon a source uses before choosing a car on it.
- Multiplying instead of dividing for L/100 km. 30 mpg × 0.425 gives 12.75, which is km/L, not L/100 km. The reciprocal step turns it into 7.84.
- Averaging mpg across trips or vehicles. Add up the fuel and the distance, then divide, or average L/100 km figures for equal distances.
- Entering zero. A car using 0 L/100 km would have infinite mpg, so the converter shows no result for a zero in L/100 km, and 0 mpg has no L/100 km figure either.
Official ratings come from standardized test cycles that differ between the US, Europe and Japan, so a converted rating is the same number in another unit, not a prediction of your own consumption. To see what a trip will cost, use the fuel cost calculator.
Questions people ask
How do I measure my own car’s fuel economy?
Fill the tank, reset the trip counter, drive normally and fill up again. Divide the distance by the fuel added for mpg or km/L, or divide the fuel by the distance and multiply by 100 for L/100 km. The gas mileage calculator does the arithmetic.
Which unit should I use to compare two cars?
Either works for a straight comparison, but L/100 km (or gallons per 100 miles) shows fuel savings in proportion, so it is the better guide to how much fuel a switch would save.
Sources
- Learn More About the Fuel Economy Label for Gasoline Vehicles, U.S. Department of Energy and U.S. Environmental Protection Agency (fueleconomy.gov).
- Weights and Measures Act 1985, Schedule 1 (definitions of units), legislation.gov.uk (The National Archives).
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811, Appendix B, National Institute of Standards and Technology.