Fuel Consumption Converter

Fuel economy versus fuel consumption

There are two opposite ways to describe how thirsty a vehicle is. Fuel economy says how far you travel on a fixed amount of fuel: miles per gallon (mpg), kilometers per liter (km/L). A higher number is better. Fuel consumption says how much fuel you burn over a fixed distance: liters per 100 km, gallons per 100 miles. A lower number is better.

The two families are reciprocals of each other, which is why converting between them is a division rather than a multiplication. That is also why a car that does 0 km/L would burn an infinite amount of fuel per distance: the converter shows "Infinity" for a zero in those cases.

Constants behind the conversions

Every conversion goes through liters per 100 km, using three exact definitions:

• 1 mile = 1.609344 kilometers
• 1 US gallon = 3.785411784 liters
• 1 UK (imperial) gallon = 4.54609 liters

From these come the two handy constants for miles per gallon:

mpg (US) = 235.2146 ÷ (L/100 km)

mpg (UK) = 282.4809 ÷ (L/100 km)

Both are written as 100 × gallon size ÷ mile length. A UK gallon is about 20% bigger than a US gallon (4.54609 ÷ 3.785411784 = 1.2009), so the same car posts a higher mpg figure on the UK scale: multiply a US mpg value by 1.2009 to get UK mpg.

Worked examples

From L/100 km to US mpg. A hatchback rated at 6.4 L/100 km: 235.2146 ÷ 6.4 = 36.75 mpg (US), or 282.4809 ÷ 6.4 = 44.14 mpg (UK).

From mpg to km per liter. A pickup that returns 18 mpg (US) travels 18 × 1.609344 ÷ 3.785411784 = 7.65 km per liter.

From km/L to gallons per 100 miles. A scooter that does 40 km/L uses 100 miles × 1.609344 ÷ 40 = 4.02 liters, which is 4.02 ÷ 3.785411784 = 1.06 US gallons per 100 miles.

Because the relationship is a reciprocal, improvements count for more at the thirsty end. Moving from 10 to 20 mpg saves 5 gallons over 100 miles, while moving from 30 to 40 mpg saves only 0.83 gallons over the same distance.

L/100 km to mpg conversion table

Pitfalls when comparing figures

• US versus UK mpg. Always check which gallon a quoted mpg figure uses. Mixing them overstates or understates a car by roughly one fifth.
• Test cycles versus real driving. Rated figures come from standard test procedures and rarely match what you get in traffic, with a roof box or in cold weather.
• Rounding at the edges. Very small or very large inputs give long decimals; the converter shows up to ten decimal places and drops trailing zeros.

To work out a trip's fuel use or cost from your own readings, use the fuel consumption calculator or the gas mileage calculator. The fuel cost calculator estimates what a journey will cost.