Both answers, every time
Enter a figure and this shows it in imperial mpg, US mpg, litres per hundred kilometres, kilometres per litre and miles per litre at once. Showing both gallons side by side is the point: seeing 35 imperial mpg and 29 US mpg next to each other makes it immediately obvious which one your car meant.
The exact constants are 3.785411784 litres to the US gallon and 4.54609 to the imperial, both defined rather than measured, and 1.609344 kilometres to the mile.
Why the arithmetic feels wrong
Fuel economy and fuel consumption are reciprocals, so the relationship is a division. That produces a result people find genuinely counter-intuitive: improving from 15 to 20 mpg saves more fuel across the same journey than improving from 30 to 50 does.
At 15 mpg a hundred miles takes 6.67 gallons; at 20 it takes 5.00 — a saving of 1.67. At 30 it takes 3.33 and at 50 it takes 2.00, saving 1.33 for a far larger apparent improvement. This is why fleet regulations are increasingly written in consumption rather than economy, and why the litres-per-hundred figure is the one to compare in.
Where each unit is used
Litres per hundred kilometres is standard across continental Europe, Australia, New Zealand, South Africa and Canada. Miles per imperial gallon is used in the United Kingdom. Miles per US gallon is used in the United States. Kilometres per litre appears in Japan, India and much of Latin America.
The awkward case is Canada, which uses L/100km officially but where imperial mpg persists in conversation, and the United Kingdom, which sells fuel in litres and quotes economy in gallons.
Things people ask
What is the formula?
L/100km equals 235.215 divided by US mpg, or 282.481 divided by imperial mpg. It is a division, which is why the conversion is not linear.
Which mpg does my car show?
A car sold in the UK shows imperial. One sold in the US shows US. There is no way to tell from the number alone, which is why both are shown here.
Why is lower better in L/100km?
Because it measures fuel used rather than distance covered. Fewer litres for the same hundred kilometres is less fuel.