Fuel Efficiency Converter
Fuel economy is quoted two incompatible ways: distance per unit of fuel, and fuel per unit of distance. They are reciprocals, not a simple ratio, which is why converting them by multiplication gives nonsense.
Formula
L/100km = 282.481 ÷ MPG (UK) · L/100km = 235.215 ÷ MPG (US)
- The scales are reciprocal, so you divide rather than multiply
- 1 MPG (UK) = 0.8327 MPG (US), because the gallons differ
40 MPG (UK) is 7.06 L/100km, and 33.3 MPG by US measure
The Two Scales Run in Opposite Directions
MPG measures miles travelled per gallon, so higher is better. L/100km measures litres burned per hundred kilometres, so lower is better. Converting between them means dividing, not multiplying:
- L/100km = 235.215 ÷ MPG (US)
- L/100km = 282.481 ÷ MPG (UK)
30 MPG (US) is 235.215 ÷ 30 = 7.84 L/100km. A car at 8 L/100km returns 235.215 ÷ 8 = 29.4 MPG (US).
The constants look arbitrary and are not: 235.215 is simply the number of litres in a US gallon multiplied out against the miles in 100 kilometres.
US and UK Gallons Are Not the Same Gallon
A UK brochure figure will always look better than the US one for the same car, and nothing about the car has changed. The Imperial gallon is 4.546 litres against the US gallon at 3.785, so it carries the vehicle 20% further:
- 1 MPG (UK) = 0.8327 MPG (US)
- 1 MPG (US) = 1.201 MPG (UK)
A car advertised at 40 MPG in the UK is doing 33.3 MPG by US measure — 7.06 L/100km either way, because the litres do not care which gallon you divide them by.
Why the Reciprocal Matters More Than It Looks
Because MPG is a reciprocal scale, equal-looking improvements are not equal savings. This is worth knowing before choosing which vehicle to replace.
Over 10,000 miles, going from 10 to 15 MPG saves 1,000 − 667 = 333 gallons. Going from 30 to 50 MPG — a far more impressive-sounding jump — saves 333 − 200 = 133 gallons.
Improving a thirsty vehicle by a few MPG saves more fuel than improving an efficient one by a lot. L/100km makes this obvious, because the numbers are already proportional to fuel burned, which is a fair part of why most of the world uses it.
Quick Reference
| MPG (UK) | L/100km | MPG (US) |
|---|---|---|
| 20 | 14.12 | 16.7 |
| 30 | 9.42 | 25.0 |
| 40 | 7.06 | 33.3 |
| 50 | 5.65 | 41.6 |
| 60 | 4.71 | 50.0 |
Frequently Asked Questions
- How do I convert MPG to L/100km?
- Divide 235.215 by the US MPG figure, or 282.481 by the UK MPG figure. The relationship is a reciprocal, so you divide rather than multiply.
- Why is UK MPG higher than US MPG for the same car?
- The Imperial gallon holds 4.546 litres against the US gallon at 3.785, so the same car travels about 20% further on it. Multiply US MPG by 1.201 for the UK figure.
- Is a lower L/100km better?
- Yes. It counts fuel burned over a fixed distance, so lower means more efficient. MPG works the other way, where higher is better.
- Why do small MPG gains on a thirsty car matter more?
- Because fuel used is proportional to the reciprocal of MPG. Going from 10 to 15 MPG saves far more fuel over the same distance than going from 30 to 50.