Fuel Economy Converter

How do you convert fuel economy units?

Multiply by a fixed factor to convert between mpg and km/L, and divide a constant by the value to convert to or from L/100km, because fuel economy is written two ways. Miles per gallon and kilometers per liter measure distance per unit of fuel, so a higher number is better. Liters per 100 km measures fuel per unit of distance, so a lower number is better.

The key formulas

• L/100km = 235.215 ÷ US mpg, and US mpg = 235.215 ÷ L/100km.
• L/100km = 282.481 ÷ UK mpg, and UK mpg = 282.481 ÷ L/100km.
• km/L = US mpg × 0.425144, and L/100km = 100 ÷ km/L.
• UK mpg = US mpg × 1.20095, because the imperial gallon holds about 20% more fuel.

For example, to convert 30 US mpg to L/100km, divide 235.215 by 30 to get 7.84 L/100km. The same car does 30 × 0.425144 = 12.75 km/L, as the mpg to km/L converter shows, and 30 × 1.20095 = 36.03 UK mpg. The mpg to L/100km converter runs this math for any figure.

US gallons versus UK gallons

A US gallon is 3.785411784 liters and an imperial gallon is 4.54609 liters. A British car quoted at 50 mpg is therefore only about 41.6 US mpg. Always check which gallon a figure uses before you compare cars from different markets; pick UK mpg in the converter at the top of this page to switch between them.

How do you calculate fuel use and the cost of a trip?

Divide the distance by the mpg figure to get gallons, or multiply L/100km by the distance in hundreds of kilometers to get liters, then multiply the fuel by its price. Use the same gallon throughout: US mpg with US gallons, UK mpg with imperial gallons.

• In miles: a 300-mile trip at 30 US mpg needs 300 ÷ 30 = 10 gallons, which is 37.85 liters.
• In kilometers: a 500 km trip at 7 L/100km needs 7 × 5 = 35 liters, about 9.25 US gallons.
• Over a year: driving 12,000 miles at 25 mpg burns 480 gallons; at 35 mpg it burns 342.86 gallons, a saving of 137.14 gallons.

To measure your own car, fill the tank, reset the trip meter, drive normally, then refill and note the fuel added. Divide miles by gallons for mpg, or divide liters by kilometers and multiply by 100 for L/100km: 42 L over 600 km is 7 L/100km. The liters to gallons converter and the km to miles converter handle mixed units, and the length converter covers any other distance.

Why is L/100km better for comparing fuel efficiency?

Because the fuel you burn is directly proportional to L/100km, while equal jumps in mpg do not save equal amounts of fuel. Mpg is distance per fuel, so each extra mpg saves less fuel than the one before. Over 10,000 miles, going from 10 to 20 mpg cuts fuel use from 1,000 to 500 gallons, a saving of 500 gallons. Going from 30 to 50 mpg cuts it from 333.3 to 200 gallons, a saving of only 133.3 gallons, even though the second jump is 20 mpg instead of 10.

Fuel-per-distance units make this obvious. In L/100km the first upgrade is 23.52 to 11.76, and the second is 7.84 to 4.70. The fuel you burn, and the money you spend on it, is directly proportional to L/100km, so you can multiply it by your yearly distance to estimate consumption. This is also why the US EPA fuel economy label shows a gallons per 100 miles figure next to the mpg rating.

Which fuel economy units are used where?

US mpg is used in the United States, UK mpg in the United Kingdom, L/100km across Europe, Canada, Australia and New Zealand, and km/L in Japan, India and much of Asia and Latin America.

• US mpg: window stickers and EPA ratings in the United States, where fuel is sold by the US gallon.
• UK mpg: car reviews and manufacturer brochures in the United Kingdom, based on the imperial gallon even though fuel is sold by the liter.
• L/100km: official ratings across Europe, Canada, Australia and New Zealand.
• km/L: Japan, India, much of Southeast Asia and Latin America, where it is read the same way as mpg. The mpg to km/L converter translates between these ratings and US figures.
• Trip planning: multiply L/100km by the distance in hundreds of kilometers to estimate the fuel you need, then use the volume converter if you want the answer in gallons.

How is electric car efficiency measured?

Electric vehicles are rated in energy per distance, kWh per 100 miles in the US and kWh per 100 km in Europe, and the US EPA also shows MPGe, miles per gallon equivalent, which counts 33.7 kWh of electricity as one gallon of gasoline. Like L/100km, a lower kWh/100 figure is better; like mpg, a higher MPGe is better.

To convert, divide 3,370 by the kWh per 100 miles to get MPGe, and divide kWh per 100 miles by 1.609344 to get kWh per 100 km. An EV that uses 30 kWh per 100 miles rates about 112.33 MPGe and uses 18.64 kWh per 100 km. Multiply the kWh figure by your electricity price to compare running costs with a gasoline car, and use the energy converter if you need the same energy in joules or BTU.

Official figures for both fuel and electric cars come from standardized laboratory test cycles: the EPA procedures behind US window stickers and fueleconomy.gov, and the WLTP (Worldwide Harmonised Light Vehicles Test Procedure) used for official ratings in the EU and UK. Real-world results vary with speed, temperature, load and driving style, so treat any rating as a basis for comparison rather than a guarantee.