MPG to L/100km Converter

How to convert mpg to L/100km

Miles per gallon tells you how far you go on a fixed amount of fuel. Liters per 100 km tells you how much fuel you burn over a fixed distance. One is distance ÷ fuel and the other is fuel ÷ distance, so you cannot convert between them by multiplying; you have to divide a constant by the value. That is why a higher mpg always means a lower L/100km. The L/100km to mpg converter runs the same division in reverse, and the fuel economy converter shows km/L and UK mpg side by side. If your figure uses imperial gallons, pick UK mpg on the Gallon switch above.

The constant comes straight from the unit definitions. A US gallon is exactly 3.785411784 liters and a mile is exactly 1.609344 km. Burning one gallon per mile would be 3.785411784 L per 1.609344 km, and scaling that to 100 km gives 100 × 3.785411784 ÷ 1.609344 = 235.2146. Divide it by the mpg figure and you have L/100km. With the larger imperial gallon of 4.54609 L the constant becomes 282.4809. The liters to gallons converter shows the two gallon sizes on their own.

Step by step

1. Check whether the figure is in US mpg or UK (imperial) mpg.
2. Divide 235.215 by US mpg, or 282.481 by UK mpg.
3. Round to one decimal place, the way official ratings are usually shown.

See it on a scale

Why does L/100km show fuel savings better than mpg?

L/100km moves in step with the fuel you actually burn, while mpg does not, so the same mpg gain saves very different amounts of fuel depending on where it starts.

Most people assume a gain of 10 mpg saves the same fuel whatever the starting point. It does not. Researchers Richard Larrick and Jack Soll called this the “mpg illusion” in a 2008 paper in Science. Look at fuel used over 100 miles:

• Going from 10 to 20 mpg cuts fuel from 10 to 5 gallons, a saving of 5 gallons.
• Going from 20 to 30 mpg cuts it from 5 to 3.33 gallons, a saving of 1.67 gallons.
• Going from 30 to 50 mpg cuts it from 3.33 to 2 gallons, a saving of only 1.33 gallons.

Replacing a 12 mpg truck with a 15 mpg one saves more fuel than replacing a 40 mpg car with a 60 mpg hybrid. Fuel-per-distance units such as L/100km make this obvious because the numbers move in step with the fuel you burn: 12 → 15 mpg is 19.6 → 15.7 L/100km, while 40 → 60 mpg is only 5.9 → 3.9. It is also why US window stickers now show gallons per 100 miles alongside mpg.

How do you average mpg across several trips?

Divide the total miles by the total gallons, or average L/100km weighted by distance; a plain average of mpg figures overstates the economy you actually got.

Because mpg is a reciprocal measure, you cannot simply average two mpg figures. Suppose you drive 100 miles at 20 mpg and another 100 miles at 40 mpg. The simple average says 30 mpg, but you actually used 5 + 2.5 = 7.5 gallons for 200 miles, which is 26.67 mpg.

Convert to L/100km first and the math behaves. 20 mpg is 11.76 L/100km and 40 mpg is 5.88 L/100km. For equal distances, their plain average is 8.82 L/100km, which converts back to exactly 26.67 mpg. Engineers and fleet managers use fuel-per-distance units for exactly this reason.

When you need mpg in L/100km

• Driving or renting in Canada, Europe or Australia. Official ratings, dashboards and car reviews show L/100km. Canada's government fuel consumption ratings use it too.
• Comparing imported cars. A European review and a US review of the same model will quote different units, and often different test cycles, so convert before you compare.
• Estimating trip fuel. Multiply L/100km by the trip distance in km and divide by 100. A 450 km trip at 7.84 L/100km uses about 35.3 L. Use the mph to km/h converter if the speed limits there are new to you too.
• Metric-market cars. For Asia and Latin America, where ratings are usually in kilometers per liter, use the mpg to km/L converter instead.