The short answer
- L/100 km = about 282.5 ÷ UK mpg, or about 235.2 ÷ US mpg.
- UK mpg is about 1.2 times US mpg for the same car.
- Your real MPG = distance since the last full tank ÷ fuel added to refill.
- Cost per mile = price per litre × litres per mile.
The four ways to measure economy
| Unit | Used in | Better is |
|---|---|---|
| Miles per UK gallon | UK | Higher |
| Miles per US gallon | US | Higher |
| Litres per 100 km | Europe, Australia, Canada | Lower |
| Km per litre | Japan, India, Latin America | Higher |
MPG and km per litre measure distance per unit of fuel. L/100 km measures fuel per unit of distance, so a smaller number means a more efficient car.
UK and US gallons
The UK (imperial) gallon is defined as exactly 4.54609 litres. The US gallon is 231 cubic inches, which is 3.785411784 litres. A UK gallon is therefore about 20% bigger, and the same car shows about 20% more miles per gallon in UK figures. A car rated at 50 mpg in the UK does about 41.6 mpg in US gallons. UK fuel is sold by the litre, but car economy is still quoted in miles per UK gallon.
Converting between units
- 50 miles × 1.60934480.47 km
- 4.54609 litres ÷ 80.47 km0.0565 L per km
- × 1005.65 L/100 km
Because L/100 km is the reciprocal of MPG, the shortcut is to divide a constant by the MPG: about 282.5 for UK mpg and 235.2 for US mpg. Km per litre is 100 ÷ L/100 km.
Conversion table
| UK mpg | US mpg | L/100 km | km/L |
|---|---|---|---|
| 30 | 25.0 | 9.42 | 10.6 |
| 36.0 | 30 | 7.84 | 12.8 |
| 40 | 33.3 | 7.06 | 14.2 |
| 48.0 | 40 | 5.88 | 17.0 |
| 50 | 41.6 | 5.65 | 17.7 |
| 60 | 50.0 | 4.71 | 21.2 |
Measuring your real MPG
The dashboard figure is an estimate. The fill-up method is more accurate:
- Fill the tank until the pump clicks off, and reset the trip meter.
- Drive as normal, ideally for at least half a tank.
- Fill up again at the same pump if you can, and note the litres added.
- Divide the distance on the trip meter by the fuel added.
- 38.5 litres ÷ 4.546098.47 UK gallons
- 412 miles ÷ 8.4748.6 mpg (UK)
Repeat over several tanks and average the results for a steadier figure.
Cost per mile and per trip
The UK average petrol price was about 173.8p a litre and diesel about 198.9p in late September 2026. At 45 mpg (UK), a car uses about 0.101 litres a mile.
| Fuel | Price a litre | Per mile | 200 miles |
|---|---|---|---|
| Petrol | 173.8p | 17.6p | £35.12 |
| Diesel | 198.9p | 20.1p | £40.19 |
In the US, with fuel priced per US gallon, divide the price by US mpg. At an example price of $3.20 a gallon and 30 mpg (US), fuel costs about 10.7 cents a mile, or $21.33 for 200 miles. For UK journeys with passengers, parking, tolls and HMRC mileage rates, use the fuel cost per journey calculator.
What better MPG saves in a year
| UK mpg | Cost a year |
|---|---|
| 30 | £2,634 |
| 40 | £1,975 |
| 50 | £1,580 |
| 60 | £1,317 |
The step from 30 to 40 mpg saves about £659 a year; the step from 50 to 60 saves about £263. To compare petrol, diesel and electric over several years, see the petrol vs EV running cost calculator.
The MPG illusion
Equal MPG gains are not equal savings
Fuel used depends on 1 ÷ MPG, so each extra mile per gallon saves less than the one before. Replacing a 30 mpg car saves far more fuel than replacing a 50 mpg one. Litres per 100 km avoids this trap, because savings scale with the number.
Official figures and real driving
Official figures come from a standard laboratory test (WLTP in the UK and Europe; the EPA test in the US). Real driving often uses more: short trips with a cold engine, motorway speeds, stop-start traffic, heavy loads, roof boxes, under-inflated tyres and air conditioning all raise fuel use. The fill-up method shows what your car really does.
Using less fuel
- Drive smoothly: gentle acceleration and early braking save fuel.
- Keep to the speed limit; fuel use rises quickly at higher speeds.
- Check tyre pressures monthly and remove roof racks when not in use.
- Combine short trips, and share lifts. The commuter cost comparison compares driving with the train, bus and bike.
Prices per litre and per gallon
UK and European pumps price fuel by the litre; US pumps by the US gallon. To compare, convert both to the same unit. 173.8p a litre is about £7.90 per UK gallon. An example US price of $3.20 a gallon is about $0.85 a litre. Choose the unit your price uses under “Price is per”, and the calculator converts it.
Driving abroad
Hiring a car abroad, you may see economy in L/100 km and distances in kilometres. Set the trip distance to kilometres under More options and the currency to euros or dollars, and enter the local price per litre. A car that does 50 mpg (UK) at home uses about 5.65 litres every 100 km, so a 400 km drive needs about 22.6 litres.