⛽ Fuel Economy Converter
Convert fuel economy between US MPG, UK (Imperial) MPG, liters per 100km and kilometers per liter.
The four units, and why they disagree
| Unit | Measures | A bigger number means |
|---|---|---|
| MPG (US) | Miles per US gallon (3.785 L) | More efficient |
| MPG (UK/Imperial) | Miles per imperial gallon (4.546 L) | More efficient |
| L / 100 km | Litres burned per 100 km | Less efficient |
| km / L | Kilometres per litre | More efficient |
Three of these are distance-per-fuel and one is fuel-per-distance. That single difference causes most of the confusion around fuel figures, and it has a consequence covered below.
The US and imperial gallon are not the same
An imperial gallon is 4.54609 litres; a US gallon is 3.785411784. The imperial one is about 20% larger, so the same car scores about 20% higher in UK MPG than in US MPG.
A car advertised at 40 MPG in Britain is roughly 33 MPG in the United States. Nothing about the car changed. This is the most common error when comparing a figure against a review written on the other side of the Atlantic.
Why you cannot average MPG
MPG and L/100 km are reciprocals, so equal steps in one are not equal steps in the other. That makes averaging MPG across two journeys or two cars quietly wrong.
Take two cars driven the same distance, one at 10 MPG and one at 50 MPG. The average is not 30 MPG. Over 100 miles the first burns 10 gallons and the second burns 2, so 200 miles cost 12 gallons, which is about 16.7 MPG.
Rough figures worth carrying in your head
- 235 divided by MPG (US) is approximately L/100 km, and the reverse works too.
- 282 divided by MPG (UK) is approximately L/100 km.
- km/L multiplied by 2.35 is approximately MPG (US).
Those are close enough for a mental check. The converter uses the exact constants, 1.609344 km per mile and the two gallon definitions above, so its answers differ slightly from the shortcuts.
Frequently asked questions
How do I convert MPG to L/100 km?
Divide 235.215 by the US MPG figure, or 282.481 by the imperial one. The relationship is a division rather than a multiplication because the two units are reciprocals of each other.
Is a higher L/100 km better?
No. It is the one unit here where lower is better, because it counts fuel consumed rather than distance travelled. 5 L/100 km is a frugal car; 15 L/100 km is a thirsty one.
Why is UK MPG higher than US MPG for the same car?
Because the imperial gallon is about 20% larger than the US gallon, so the car covers more miles on one of them. The difference is in the unit, not in the vehicle.
What counts as good fuel economy?
For a modern petrol car, roughly 6 to 8 L/100 km, which is about 30 to 40 MPG (US). Hybrids commonly reach 4 to 5 L/100 km. Large trucks and SUVs sit above 12 L/100 km.
Can I average two trips to get overall economy?
Not by averaging MPG. Add the total distance and the total fuel, then divide once. Averaging in L/100 km is safe when both trips are the same distance, because that unit is linear in fuel.