How Fuel Economy Conversion Works
Fuel economy is expressed differently around the world. The United States uses miles per gallon (US gallon). The United Kingdom and Commonwealth countries also use miles per gallon, but with the larger imperial gallon. Europe, Asia, and most of the rest of the world use litres per 100 kilometres (L/100km). Japan and a few others use km/L.
Converting between these units requires knowing that the US gallon (3.785 L) and UK gallon (4.546 L) are different sizes, and that MPG and L/100km have an inverse relationship — better fuel economy means higher MPG but lower L/100km. This calculator handles all conversions in real time.
Fuel Economy Conversion Factors
- 1 MPG (US) = 1.20095 MPG (UK)
- 1 MPG (US) × L/100km = 235.215 (inverse relationship)
- 1 MPG (US) = 0.425144 km/L
- 1 L/100km = 100 km/L⁻¹ (inverse of km/L × 100)
Understanding CO₂ Emissions from Fuel Economy
Petrol (gasoline) combustion produces approximately 2.31 kg of CO₂ per litre burned. This means fuel economy and CO₂ emissions are directly linked — a car that burns more fuel per distance produces proportionally more CO₂. The CO₂ estimate in this calculator uses L/100km as the common basis for comparison.
- EU average new car CO₂ target: 95 g/km (equates to ~4.1 L/100km)
- Typical petrol car: 120–180 g CO₂/km
- Fuel-efficient hybrid: 80–100 g CO₂/km
- Battery electric vehicle: 0 g/km direct emissions (lifecycle depends on electricity source)
Frequently Asked Questions
Why is UK MPG higher than US MPG for the same car?
The UK (imperial) gallon is larger than the US gallon — 4.546 litres versus 3.785 litres. So a car that travels the same distance using the same amount of fuel will show a higher MPG number in the UK system because you are dividing miles by a larger gallon size. A car rated at 40 MPG (US) is approximately equivalent to 48 MPG (UK).
Is lower L/100km better or worse?
Lower L/100km is better — it means you are using fewer litres to travel 100 kilometres, so the car is more fuel efficient. This is the opposite of MPG, where higher numbers mean better efficiency. A car rated at 5.0 L/100km is more efficient than one rated at 8.0 L/100km.
How accurate is the CO₂ estimate?
The estimate assumes average petrol combustion at 2.31 kg CO₂/litre. Real-world emissions vary slightly based on fuel blend, driving conditions, engine efficiency, and whether short trips prevent the engine from reaching optimal temperature. Official vehicle CO₂ ratings are measured under standardised test conditions (WLTP in Europe, EPA in the US) and may differ from real-world driving.