How To Convert Fuel Economy

Fuel economy measures how far a vehicle travels on a given amount of fuel. Different countries express it in different units, and not all of them point the same way.

Miles per gallon and kilometres per litre are distance-per-fuel units: a larger number means better economy. Miles per gallon comes in US and UK versions because the two gallons differ.

The US gallon is 3.785411784 litres and the UK imperial gallon is 4.54609 litres. A UK mile per gallon is therefore about 20 percent larger than a US mile per gallon for the same real economy.

Litres per 100 kilometres is a fuel-per-distance unit: a smaller number means better economy. Because it is the reciprocal of the distance-per-fuel units, you cannot convert to it with a simple multiplication.

The converter uses kilometres per litre as the base for the distance-per-fuel units. Each unit has a fixed size in kilometres per litre, so the conversion multiplies by the source factor and divides by the target factor.

One mile is 1.609344 kilometres, so one mile per litre is 1.609344 kilometres per litre. One US mile per gallon is 1.609344 divided by 3.785411784, about 0.425143707 kilometres per litre.

One UK mile per gallon is 1.609344 divided by 4.54609, about 0.354006 kilometres per litre, which is why a UK figure is always higher than the US figure for the same car.

The litres per 100 kilometres figure is found by dividing 100 by the kilometres per litre value. So one US MPG gives 100 divided by 0.425143707, about 235.2145833 litres per 100 kilometres.

Because the relationship is inverted, improving fuel economy lowers the litres per 100 kilometres figure. A car that goes from 8 to 6 litres per 100 kilometres is using a quarter less fuel, even though the number fell by only a quarter.

A common mistake is to convert between miles per gallon and litres per 100 kilometres by multiplying. The correct operation divides, because one is the reciprocal of the other.

A second trap is to ignore the difference between US and UK gallons. Using the wrong gallon changes the answer by about twenty percent, which is large enough to mislead a purchase decision.

A car advertised at 30 miles per gallon in the United States is about 36 miles per gallon in UK terms, because the UK gallon is larger. The same car has one real economy and two numbers.

European figures are usually litres per 100 kilometres, so a smaller number is better. A car at 6 litres per 100 kilometres uses less fuel than one at 8, even though the number is lower.

Comparing running costs across regions needs a common unit. Converting everything to litres per 100 kilometres makes the comparison straightforward, since fuel is priced per litre in most places.

Electric vehicles are often quoted in kilowatt-hours per 100 kilometres, the direct analogue of litres per 100 kilometres. The same inverted logic applies, with lower being better.

Real-world economy rarely matches the published figure. Traffic, cold starts and air conditioning all reduce it, so treat any conversion as a best case rather than a guarantee.

Trip planning needs the same conversion in reverse. If a journey is 400 kilometres and the car uses 6 litres per 100 kilometres, it needs about 24 litres, which is a straightforward use of the fuel-per-distance form.

Fleet operators convert everything to a single unit so vehicles can be compared across regions. Litres per 100 kilometres is the common denominator, since it scales directly with distance.

The calculator models the fixed relationships between the units and nothing more. It does not know your real driving conditions, so treat the output as exact arithmetic on the number you entered.