Fuel economy is the one conversion on this site that cannot be done by multiplying, because the two systems measure opposite things. Miles per gallon is distance per fuel: higher is better. Litres per 100 km is fuel per distance: lower is better. They are reciprocals, so converting means dividing rather than scaling.
The formula. L/100km = 235.215 ÷ mpg (US),
and the same division goes back the other way. For imperial mpg the constant
is 282.481. If you ever multiply, the answer will be wrong.
Quick reference
The table follows whichever units you select above.
Two different gallons, again
An imperial gallon is about 20 per cent larger than a US one, so the same car reports noticeably better economy in British figures. A car doing 30 mpg in the United States does 36 mpg in the United Kingdom without anything changing but the unit.
This makes cross-Atlantic comparisons of "miles per gallon" meaningless unless the gallon is specified — one reason so much of the world moved to L/100 km, where there is only one litre and only one kilometre.
Why the reciprocal matters more than it sounds
Because economy is inverted, equal steps in mpg are not equal savings. Improving from 10 to 15 mpg saves far more fuel over the same distance than improving from 30 to 35 — the same five-mpg step.
| Change | Fuel for 10,000 mi | Saved |
|---|---|---|
| 10 → 15 mpg | 1,000 → 667 gal | 333 gal |
| 20 → 25 mpg | 500 → 400 gal | 100 gal |
| 30 → 35 mpg | 333 → 286 gal | 47 gal |
| 40 → 45 mpg | 250 → 222 gal | 28 gal |
Researchers call this the MPG illusion. Replacing a very thirsty vehicle with a merely inefficient one saves more fuel than upgrading an already efficient car to a slightly better one, even though the mpg gain looks identical. Consumption units make this obvious; economy units hide it. It is the strongest practical argument for L/100 km.
Common questions
How do I convert mpg to L/100km?
Divide 235.215 by the US mpg figure. For imperial mpg, divide 282.481. The same division converts back, which is a useful property of reciprocals.
Is 7 L/100km good?
It is about 33.6 mpg (US) or 40.4 mpg (imperial) — respectable for a petrol family car, unremarkable for a small diesel, poor for a hybrid.
Why do US and UK mpg figures differ for the same car?
The imperial gallon is 4.546 L against 3.785 L for the US gallon, so the imperial figure is about 20 per cent higher. Official test cycles differ too, which widens the gap further.