UK imperial gallons by default — 4.54609 litres. Useful when comparing a UK car with European or US economy figures.
L/100km = 282.481 ÷ MPG (UK). The constant comes from 454.609 ÷ 100 × 62.137 — the conversion between imperial gallons, litres and miles to kilometres. For US gallons the constant is 235.215, which is why the same car looks more efficient on an American window sticker.
| MPG (UK) | L/100km | MPG (US) |
|---|---|---|
| 30 | 9.4 | 25.0 |
| 35 | 8.1 | 29.1 |
| 40 | 7.1 | 33.3 |
| 45 | 6.3 | 37.4 |
| 50 | 5.6 | 41.6 |
| 55 | 5.1 | 45.8 |
| 60 | 4.7 | 49.9 |
They are inverse measures, which makes mental arithmetic unreliable — a car going from 50 to 55 MPG saves less fuel than one going from 25 to 30 MPG, even though both are "5 MPG better". L/100km shows that honestly: 50→55 MPG is 5.6→5.1 L/100km (0.5 litres saved), while 25→30 MPG is 11.3→9.4 L/100km (1.9 litres saved).
The UK still quotes economy in imperial gallons (4.54609 litres) even though fuel is sold in litres. A US gallon is 3.785 litres, so a US MPG figure is about 20% lower for the same real economy. If you are comparing a UK car to a US review, convert to L/100km first — it is the only unit both sides agree on.
The main calculator handles petrol, diesel and EV side by side, with cost per mile and CO₂.
45 MPG (UK) is 6.3 L/100km. In US gallons the same economy is 37.4 MPG.
No. A UK imperial gallon is 4.546 litres against 3.785 in the US, so UK MPG figures are about 20% higher for identical fuel economy.