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Fuel economy is the one category that works by inverse rather than by ratio, and that is exactly why people get it wrong. Miles per gallon rises as a car becomes more efficient; liters per 100 km falls. The two are reciprocals, so no single multiplier turns one into the other - you have to divide.
The second trap is which gallon. The same car returns 30 mpg US, 36 mpg UK and 7.84 L/100km, and mixing the two gallons introduces a 20% error into a figure people compare across countries. Both conversions run through liters per 100 km: US mpg is 235.2145833 divided by L/100km, and UK mpg is 282.4809363 divided by the same figure.
| mpg US | mpg UK | L/100km | km/L | |
|---|---|---|---|---|
| 1 mpg US | 1 | 1.20095 | 235.214583 | 0.425144 |
| 1 mpg UK | 0.832674 | 1 | 282.480936 | 0.354006 |
| 1 L/100km | 235.214583 | 282.480936 | 1 | 100 |
| 1 km/L | 2.352146 | 2.824809 | 100 | 1 |
How do I convert mpg to L/100km? Divide 235.2145833 by the mpg figure for US gallons, or 282.4809363 for imperial gallons. So 30 mpg US is 235.2145833 / 30 = 7.84 L/100km. Multiplying instead of dividing is the most common mistake in this category.
Why does a higher mpg go with a lower L/100km? Because they run in opposite directions by design. Higher mpg means more distance per unit of fuel; lower L/100km means less fuel per fixed distance. Both describe efficiency - you just have to know which way the number should move.
What is the difference between US and UK mpg? About 20%, because the gallons differ: 3.785 L against 4.546 L. The same real consumption reads 36 mpg UK when it reads 30 mpg US, so always check which gallon a figure uses.
Can I convert 0 mpg or 0 L/100km? No, and the converter will refuse. Zero consumption in one direction means infinite in the other, because the relationship is a division. Both units are meaningful only above zero.