MPG vs L/100 km: Why Fuel Economy Numbers Never Line Up
Higher MPG is better; lower L/100 km is better. The two are reciprocals, which is why the same car scores 30 MPG in the US and 36 in the UK.
9 February 2026 · 6 min read · Automotive
Fuel economy is the trickiest everyday conversion, because two independent things differ at once: the size of the gallon, and the direction of the ratio.
Problem one: two gallons
- US gallon = 3.785411784 litres
- Imperial (UK) gallon = 4.54609 litres
The imperial gallon is 20% bigger. A car that does 30 MPG in America does 36 MPG in Britain without a single thing changing about the car. Any MPG figure without a flag next to it is incomplete. Use the US MPG to UK MPG converter to compare like with like.
Problem two: the ratio runs backwards
MPG is distance per fuel — bigger is better. L/100 km is fuel per distance — smaller is better. Converting between them means inverting:
L/100 km = 235.215 ÷ MPG (US) MPG (US) = 235.215 ÷ L/100 km
For imperial MPG the constant is 282.481.
Why this makes the graph curve
Plot MPG against L/100 km and you get a hyperbola, not a straight line. That has a real consequence people find counterintuitive:
| Upgrade | Fuel saved over 10,000 km |
|---|---|
| 10 → 12 MPG | 156 litres |
| 30 → 32 MPG | 49 litres |
| 40 → 42 MPG | 28 litres |
Improving a thirsty vehicle saves far more fuel than improving an efficient one, even though both gained 2 MPG. Replacing a company van's 10 MPG with 12 MPG saves three times the fuel of taking a hatchback from 30 to 32. The fuel economy converter plots this curve for you.
Quick reference
| L/100 km | MPG (US) | MPG (UK) | km/L |
|---|---|---|---|
| 4 | 58.8 | 70.6 | 25.0 |
| 6 | 39.2 | 47.1 | 16.7 |
| 8 | 29.4 | 35.3 | 12.5 |
| 10 | 23.5 | 28.2 | 10.0 |
| 12 | 19.6 | 23.5 | 8.3 |
Electric cars broke the units again
EVs are rated in kWh/100 km, miles per kWh, or the American MPGe — which converts electricity to a "gasoline equivalent" at 33.7 kWh per gallon. MPGe is useful for comparing an EV to a petrol car, and useless for comparing two EVs. For those, use kWh/100 km.
Related conversions
This article is for general information. Please verify figures against the relevant standard before using them in critical work. About accuracy