Two kinds of fuel economy, and why one needs a reciprocal
Miles per gallon and kilometers per liter answer the question how far does one unit of fuel take me? Liters per 100 kilometers asks the opposite: how much fuel does a fixed distance cost? That makes L/100 km the reciprocal of the others, so a car that improves from 10 to 5 L/100 km has doubled its km/L, from 10 to 20.
- Within a kind it is a straight multiplication. 1 mpg (US) is 0.4251437 km/L, and 1 mpg (UK) is 0.3540062 km/L.
- Between kinds you divide a constant by your number: L/100 km = 100 ÷ km/L. For miles per US gallon the constant is 235.2145833, and for UK gallons it is 282.4809363.
- The sum works both ways. The same constant turns L/100 km into mpg, so 7.84 L/100 km is 30 mpg (US).
The constants come from the size of the gallon and the mile: 100 × 3.785411784 ÷ 1.609344 for US gallons, and 100 × 4.54609 ÷ 1.609344 for UK ones. The mpg to L/100 km page has a table for the most common direction.
US gallons, UK gallons and which unit is used where
A US gallon is 3.785411784 liters and a UK (imperial) gallon is 4.54609 liters, about 20% bigger. The same car therefore has a bigger mpg number in UK units: 30 mpg (US) is 36.03 mpg (UK). A figure with no label is ambiguous, and you cannot fix it by looking at the number alone. American window stickers use US gallons, and figures quoted in the United Kingdom use UK ones.
Many countries have dropped gallons. Europe, Canada, Australia and China commonly quote liters per 100 km, and Japan and India commonly quote kilometers per liter. Which one you meet depends on where the figure comes from.
The mpg illusion
Because mpg is the reciprocal of fuel used, equal steps in mpg do not save equal fuel. Going from 10 to 20 mpg saves 500 US gallons over 10,000 miles, since the car goes from using 1,000 gallons to 500. Going from 30 to 40 mpg, which is also a gain of 10, saves only 83.3 gallons. The 10 to 20 change is the far bigger saving.
Fuel per distance does not have this trap, because the savings are proportional to the change. This is the usual argument for rating cars in L/100 km, and it is why comparing two cars in mpg can mislead. Convert to L/100 km first.
What this converter does not cover
It converts a number you give it. It does not predict what your car will do on the road, and a figure from a test cycle is usually more optimistic than what you get in daily driving. It does not handle electric or plug-in hybrid efficiency such as kWh per 100 km or MPGe, which measure energy rather than fuel volume.