Why this is not a multiplication
Every other conversion on this site multiplies. This one divides, because the two units measure opposite things. Miles per gallon is distance per unit of fuel, so higher is better. Litres per 100 km is fuel per unit of distance, so lower is better.
The consequence is that the relationship is inverse: doubling mpg halves L/100km. A car at 30 US mpg uses 7.84 L/100km; at 60 mpg it uses 3.92. Multiplying by a fixed factor cannot produce that behaviour, and treating this as a linear conversion is the most common error in fuel economy comparison.
Which gallon, and why UK figures look flattering
A UK gallon is about 20% larger than a US gallon, so a car travels further on one. British fuel economy figures are therefore about 20% higher than American ones for identical real-world efficiency: a car doing 40 mpg in the UK is doing about 33 mpg in the US.
This is not a rounding difference and it is not a difference in the car. Comparing a British review against an American one without switching the toggle above will make the same vehicle appear substantially more efficient in Britain, and the mistake is common enough to appear in published comparisons.
Why Europe measures fuel the other way round
L/100km looks backwards to anyone raised on mpg, but it has a real advantage: it is proportional to what you actually spend. Fuel used, and therefore cost, scales linearly with L/100km, so a car at 10 L/100km costs exactly twice as much to run per kilometre as one at 5.
Mpg does not behave that way, and this is the source of a genuine and widely misunderstood effect. Improving from 10 to 15 mpg saves far more fuel over the same distance than improving from 30 to 35 mpg, despite both being a five-mpg gain. Over 10,000 miles, the first saves 167 gallons and the second saves 48. Judging efficiency gains by mpg alone systematically undervalues improvements at the thirsty end of the range — which is why replacing an old truck matters more than upgrading an already-efficient car.
Reading the ratings sceptically
Official figures come from standardised test cycles, not from driving, and real-world consumption is routinely worse. Cold weather, short journeys, roof boxes, underinflated tyres, air conditioning and aggressive acceleration all cost fuel, and electric range figures are affected even more sharply by temperature.
The honest use of a manufacturer's figure is comparative rather than absolute: it tells you which of two cars is more efficient under identical test conditions, not what either will cost you. For that, track your own consumption over several tanks and convert the result. The litres to gallons converter handles the volume side if your pump and your dashboard disagree.
Frequently asked questions
Why is mpg to L/100km a division rather than a multiplication?
Because the units are inverses. Mpg is distance per fuel and L/100km is fuel per distance, so doubling one halves the other. L/100km equals 235.215 divided by US mpg.
Why do UK cars have higher mpg figures than US ones?
The Imperial gallon is about 20% larger, so a car travels further on one. A car doing 40 mpg in the UK is doing about 33 mpg in the US for identical real-world efficiency.
Is going from 30 to 35 mpg the same saving as 10 to 15 mpg?
No, and it is far smaller. Over 10,000 miles, improving from 10 to 15 mpg saves 167 gallons while improving from 30 to 35 saves 48. Mpg gains are worth much more at the thirsty end.