Fuel Economy Converter

How the Fuel Economy Converter works

MPG to L/100km, and why the two scales run backwards

Miles per gallon measures distance per unit of fuel: bigger is better. Litres per 100 km measures fuel per unit of distance: smaller is better. They are reciprocals, so a car review from Germany and one from Detroit describe the same engine with numbers that move in opposite directions. Add the fact that a UK gallon holds 4.546 litres and a US gallon 3.785, and a British "50 mpg" hatchback turns out to be a 41.6 mpg car on an American sticker. This page converts between all four common units, so you can put two cars from two markets side by side.

The formula

Everything goes through litres per 100 km:

• L/100km from km/L: 100 ÷ km/L
• L/100km from US MPG: 235.215 ÷ MPG
• L/100km from UK MPG: 282.481 ÷ MPG

The constants are 100 × litres per gallon ÷ km per mile, using the exact legal definitions: 1 mile = 1.609344 km, 1 US gallon = 3.785411784 L, 1 imperial gallon = 4.54609 L. Because the relationship is a division, the same constant converts in both directions: 235.215 ÷ 8 L/100km = 29.4 US mpg. Between the two MPGs, UK MPG = US MPG × 1.20095.

Worked examples

Example 1: a US sedan rated 30 mpg. 235.215 ÷ 30 = 7.84 L/100km, or 12.75 km/L, and 30 × 1.201 = 36.0 UK mpg. A European buyer would compare that 7.84 against the 5 to 6 L/100km typical of a small diesel.

Example 2: a UK hatchback quoted at 52 mpg. 282.481 ÷ 52 = 5.43 L/100km, which is 43.3 US mpg. So how many litres does a 500 km trip need? 500 × 5.43 ÷ 100 = 27.2 L. Scale that by your pump price in the fuel cost calculator. L/100km converted to km/L and both miles-per-gallon scales

Under 4 L/100km is hybrid or small-diesel territory; 8 to 10 is a typical petrol SUV; 15 and above is a large pickup towing or a V8 in town.

How to read the result

The result lists the three units you did not enter, each rounded to two decimals. L/100km and km/L are the same figure inverted. MPG (US) and MPG (UK) always differ by the ratio of the gallons, so if the two ever look equal you have a unit mix-up upstream. The table under the result steps through a range in whichever unit you chose, so a buyer thinking in MPG sees round MPG values down the side.

Where each unit is official

• L/100km: the EU, Australia, Canada, China and most of South America. EU and UK figures come from the WLTP test cycle, which replaced the older NEDC in 2017 to 2019 and raised quoted consumption by roughly 20%.
• US MPG: the EPA window sticker, which since the 2013 model year also shows gallons per 100 miles, a direct fuel-per-distance figure.
• UK MPG: the UK still advertises in imperial MPG even though fuel is sold in litres.
• km/L: Japan, India, parts of Southeast Asia and Latin America.

Tips and pitfalls

• The MPG illusion. Because MPG is a reciprocal, equal steps in MPG are not equal savings. Swapping a 15 mpg truck for a 20 mpg one saves 1.67 US gallons every 100 miles; going from 40 to 50 mpg saves only 0.5. Larrick and Soll showed in Science in 2008 that most people rank those the wrong way. Fuel per distance, L/100km or gallons per 100 miles, makes the saving linear.
• Official is not real-world. Test cycles are driven gently at mild temperatures. Winter, motorway speeds above 110 km/h, roof boxes and short trips can add 15 to 30%.
• Brim-to-brim is the honest measure. Fill the tank, reset the trip meter, drive, refill, and divide litres by kilometres. Dashboard displays often read a few percent optimistic.
• Check which gallon. Canadian and older British sources quote imperial MPG; US forums quote US MPG. If a figure looks 20% too good, it is probably imperial.

To convert the distances or volumes themselves, use km to miles and litres to gallons. If you are comparing against an electric car, the electricity cost calculator prices the kilowatt-hours.

Sources: fueleconomy.gov.

Checked against: 30 mpg (US) = 7.84 L/100 km

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