km/L to L/100 km

The formula

L/100km = 100 ÷ km/L
km/L    = 100 ÷ L/100km

The same division both ways, because the two are reciprocal. The 100 is exact — it is just the “per 100 km” in the unit’s name.

They agree at exactly one point

10 km/L is 10 L/100 km. Everywhere else they diverge, and the divergence is not symmetric:

Reading one as the other is only harmless at 10, which is unfortunate because 10 is close to the middle of the ordinary range and makes the mistake look plausible.

Where km/L is the working unit

Japan, Taiwan, Korea, India and much of Southeast Asia quote fuel economy in kilometres per litre. Europe uses L/100 km; the US and UK use mpg. So a Japanese-market car has a km/L figure on its specification and a L/100 km figure on its European one, for the same car:

Taiwan’s official figures are published in km/L, tested on a cycle of urban, highway and combined driving, and quoted as three separate numbers — the combined one is what advertising uses.

km/L is an economy unit, so it has the same illusion as mpg

Because bigger is better, equal-looking gains are worth unequal amounts of fuel. Over 10,000 km:

Each step adds five km/L and saves less than the one before. Converting to L/100 km makes that visible; leaving it in km/L hides it. See the MPG illusion on the fuel economy converter.

Motorcycles and scooters

km/L is also the unit for two-wheelers, where the numbers are much higher and the conversion correspondingly small:

Estimating in your head

Divide 100 by the number. 12.5 km/L → 100 ÷ 12.5 = 8 L/100 km, exactly. The arithmetic is easier than any other conversion in this category, because the constant is 100.