MPG vs L/100km: The Right Way to Compare UK and EU Fuel Economy Figures
UK cars are quoted in MPG using the Imperial gallon (4.546 litres). EU cars use L/100km. The conversion is not obvious — and using US MPG figures by mistake makes every car look worse than it is. Here's the correct formula and a full conversion table.
Why UK MPG Confuses Everyone
There are three different MPG standards in common use globally:
| Standard | Gallon size | Region | Notes |
|---|---|---|---|
| UK MPG (Imperial) | 4.546 litres | UK | What UK car specs use |
| US MPG | 3.785 litres | USA | About 20% lower than UK MPG for same car |
| L/100km | — | EU, most of world | Lower number = more efficient |
The UK Imperial gallon is 4.546 litres — not 3.785 litres. This single fact explains why a Ford Focus advertised at 55 UK MPG and the same model on an American website showing 46 MPG are talking about the same car.
The Conversion Formulas
UK MPG → L/100km
L/100km = 282.48 ÷ UK MPG
Where does 282.48 come from?
- 1 UK gallon = 4.546 litres
- 1 mile = 1.60934 km
- 100 km ÷ 1.60934 km/mile = 62.137 miles per 100 km
- 62.137 miles per 100 km ÷ (4.546 litres/gallon) = 13.670 gallons per 100 km
- 13.670 × 4.546 = 62.137 litres per 100 km (that's not right — let me redo)
Actually: L/100km = (100 miles/km × litres/gallon) ÷ MPG
= (100 ÷ 1.60934) × 4.546 ÷ MPG = 62.137 × 4.546 ÷ MPG = 282.48 ÷ MPG ✓
L/100km → UK MPG
UK MPG = 282.48 ÷ L/100km
The same constant works in both directions because the relationship is reciprocal.
UK MPG → US MPG
US MPG = UK MPG × (3.785 ÷ 4.546) = UK MPG × 0.8327
A car doing 60 UK MPG = 60 × 0.8327 = 49.9 US MPG
Full Conversion Table
| UK MPG | L/100km | US MPG | Annual fuel cost (10k miles)* |
|---|---|---|---|
| 25 | 11.3 | 20.8 | £2,482 |
| 30 | 9.4 | 25.0 | £2,069 |
| 35 | 8.1 | 29.1 | £1,773 |
| 40 | 7.1 | 33.3 | £1,550 |
| 45 | 6.3 | 37.5 | £1,378 |
| 50 | 5.6 | 41.6 | £1,238 |
| 55 | 5.1 | 45.8 | £1,125 |
| 60 | 4.7 | 49.9 | £1,034 |
| 65 | 4.3 | 54.1 | £953 |
| 70 | 4.0 | 58.3 | £884 |
| 80 | 3.5 | 66.6 | £774 |
At 135p/litre pump price, 10,000 miles per year.
Real-World Fuel Economy vs Manufacturer Figures
Car manufacturers must quote fuel economy under standardised test conditions. In the UK and EU, this has been WLTP (Worldwide Harmonised Light Vehicle Test Procedure) since:
- New cars: From 1 September 2019
- All new registrations: From 1 January 2021
Before WLTP, the NEDC (New European Driving Cycle) was used — a test so unrealistic that real-world fuel consumption was typically 30–40% worse than the claimed figure.
WLTP vs real-world: typical gap
| Driving type | Gap between WLTP and actual |
|---|---|
| Motorway at 70 mph (constant speed) | Close to WLTP (−5% to −10%) |
| Mixed suburban driving | −10% to −20% |
| Urban stop-start | −20% to −30% |
| Cold weather operation | −15% to −25% (petrol), worse for EVs |
| Air conditioning running | −5% to −15% |
Example: A car quoted at 60 MPG WLTP will realistically achieve 48–54 MPG in mixed real-world UK driving.
WLTP Phases: Low, Medium, High, Extra High
WLTP tests four driving phases, and manufacturers must publish a combined figure plus individual phase figures:
| Phase | Speed | Represents |
|---|---|---|
| Low | Up to 56 km/h | Urban stop-start |
| Medium | 56–76 km/h | Suburban |
| High | 76–97 km/h | Rural/A-road |
| Extra high | 97–131 km/h | Motorway |
The combined figure used in advertising is a weighted average of all four phases. Motorway-only driving will generally be better than combined (for petrol/diesel); urban-only will be worse.
Hybrid and PHEV Figures: Especially Misleading
Plug-in hybrid (PHEV) fuel economy figures are particularly misleading under WLTP because:
- Test assumes the battery starts fully charged
- The test cycle is short enough that the car may run entirely on electric power
- PHEVs are rated as low as 1–2 L/100km (130+ MPG) when battery is full
- Real-world PHEV users who rarely charge the battery see figures closer to a conventional equivalent
PHEV real-world rule: If you charge regularly and have a short commute, the WLTP figure is meaningful. If you rarely charge, treat the fuel economy as equivalent to the non-hybrid petrol version — or worse (due to added battery weight).
Comparing Electric and Petrol Cost Per Mile
EVs are measured in kWh per mile (or miles per kWh), not MPG. To compare with petrol:
| Metric | Typical figure | Cost per mile* |
|---|---|---|
| Petrol car (40 MPG UK) | 7.1 L/100km | 13.5p/mile |
| Diesel car (55 MPG UK) | 5.1 L/100km | 9.8p/mile |
| Hybrid (45 MPG UK real) | 6.3 L/100km | 12.0p/mile |
| EV (4 miles/kWh) | 0.25 kWh/mile | 6.3p/mile (at 25p/kWh home) |
| EV (4 miles/kWh) | 0.25 kWh/mile | 18.8p/mile (at 75p/kWh rapid charger) |
Petrol at 135p/litre, home EV at 25p/kWh (typical off-peak), rapid charger at 75p/kWh.
The gap between home-charged EV and petrol is significant; the gap between rapid-charged EV and petrol is much narrower or can reverse.
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