Fuel Efficiency Converter
Convert between UK MPG, US MPG, litres per 100km (L/100km), and km per litre — the four main fuel economy metrics.
Reviewed by Richard Ross · Last updated April 2026
How Fuel Efficiency Converter works
UK MPG vs L/100km
UK MPG uses the Imperial gallon (4.54609 litres). To convert: L/100km = 282.481 ÷ UK MPG. A typical UK petrol car achieves 35–50 mpg (5.6–8.1 L/100km).
UK MPG vs US MPG
The UK (Imperial) gallon is 20% larger than the US gallon. 1 UK MPG = 1.20095 US MPG. A car rated 40 UK MPG would be rated 33.3 US MPG.
L/100km interpretation
Lower L/100km means better fuel economy (less fuel per distance). Higher MPG means better fuel economy (more distance per fuel). The EU uses L/100km; UK car specs often show both.
Real-world UK fuel economy benchmarks
UK official fuel economy figures (WLTP test) are more realistic than the older NEDC figures, but still tend to be 10–20% better than real-world driving. A small petrol hatchback (e.g. Ford Fiesta) typically achieves 40–50 UK MPG in mixed driving. A diesel SUV might achieve 45–55 MPG. Hybrids commonly achieve 50–70 MPG on combined routes. Electric vehicles are rated in miles per kWh rather than MPG.
Calculating running costs from fuel economy
To estimate fuel cost per mile, divide the current petrol price (pence per litre) by the fuel economy in km/L, then convert. At 150p/litre and 14 km/L (39 UK MPG): cost per km = 150 ÷ 14 = 10.7p/km = 17.2p/mile. For a 10,000-mile annual mileage, annual fuel cost ≈ £1,720. This makes the fuel efficiency converter directly useful alongside a fuel cost calculator.
Precision and inverse relationships
MPG and L/100km are inversely related — neither can be averaged arithmetically. If you drive equal distances at 30 MPG and 50 MPG, your average is not 40 MPG. The correct combined figure is the harmonic mean: 2 ÷ (1/30 + 1/50) = 37.5 MPG. Similarly, averaging L/100km figures requires weighting by distance driven, not a simple arithmetic average.
Frequently asked questions
How do I convert MPG to L/100km?
For UK MPG: divide 282.481 by the MPG figure. Example: 40 UK MPG = 282.481 ÷ 40 = 7.06 L/100km.
What is the difference between UK and US MPG?
UK MPG uses the Imperial gallon (4.546 L); US MPG uses the US gallon (3.785 L). UK MPG figures are ~20% higher than US MPG for the same car.
What is a good MPG for a UK car?
Most petrol cars achieve 30–50 mpg. Diesel cars typically achieve 40–65 mpg. Hybrids can achieve 50–70+ mpg. Electric cars don't use MPG — their efficiency is measured in miles per kWh.
How do I convert L/100km to MPG?
UK MPG = 282.481 ÷ L/100km. Example: 6 L/100km = 282.481 ÷ 6 = 47.1 UK MPG.
Why do EU car specs list L/100km but UK specs show MPG?
The EU standardised on L/100km as part of harmonising vehicle type approval and emissions data. The UK retained MPG after independence from EU standards. Car manufacturers publishing UK specs show MPG to match consumer expectations, even though the underlying test (WLTP) produces L/100km data first.
How do I compare electric vehicle efficiency to petrol MPG?
Electric vehicles are rated in miles per kWh (or kWh per 100 km). A rough MPG equivalent (MPGe) divides 33.7 kWh (the energy in a US gallon of petrol) by the vehicle's kWh per mile. For UK use, the key metric is pence per mile: at 30p/kWh and 4 miles/kWh, an EV costs 7.5p/mile vs a 40-mpg petrol car at 150p/litre costing ~17p/mile.
Related calculators