Understanding Fuel Economy: MPG, L/100km & km/L
Convert and understand fuel economy units worldwide. Essential for international drivers and car comparisons.
Fuel economy measurements vary dramatically around the world. The US uses miles per gallon (MPG), Europe uses litres per 100 kilometres (L/100km), and some countries use kilometres per litre (km/L). Understanding these differences is essential for international drivers and car buyers.
Understanding the Different Systems
Fuel economy can be expressed two ways: distance per fuel unit or fuel per distance unit:
- MPG (miles per gallon): Higher is better. Used in US and UK.
- km/L (kilometres per litre): Higher is better. Used in parts of Asia and Latin America.
- L/100km (litres per 100 kilometres): Lower is better. Used in Europe, Canada, and Australia.
The key difference: MPG and km/L measure efficiency (more distance per fuel), while L/100km measures consumption (less fuel per distance).
US vs UK Gallons
This is a critical distinction many people miss:
- US gallon: 3.785 litres
- UK (Imperial) gallon: 4.546 litres
The UK gallon is about 20% larger, which means UK MPG figures are always higher than US MPG for the same car. A car rated at 40 US MPG would be rated at approximately 48 UK MPG.
Common Conversions
Quick reference for fuel economy conversions:
- 30 US MPG = 36 UK MPG = 7.84 L/100km = 12.75 km/L
- 40 US MPG = 48 UK MPG = 5.88 L/100km = 17.0 km/L
- 50 US MPG = 60 UK MPG = 4.70 L/100km = 21.25 km/L
- 5 L/100km = 47 US MPG = 56.5 UK MPG = 20 km/L
- 10 L/100km = 23.5 US MPG = 28.3 UK MPG = 10 km/L
Conversion Formulas
For precise calculations:
- US MPG to L/100km: 235.215 ÷ US MPG
- UK MPG to L/100km: 282.481 ÷ UK MPG
- L/100km to US MPG: 235.215 ÷ L/100km
- L/100km to UK MPG: 282.481 ÷ L/100km
- km/L to L/100km: 100 ÷ km/L
Real-World Applications
Understanding fuel economy conversions helps when:
- Renting a car abroad: Understanding local fuel economy ratings
- Buying an imported car: Comparing specs from different markets
- Reading reviews: International car reviews may use different units
- Calculating trip costs: Estimating fuel expenses in unfamiliar units
Why L/100km Is Actually Better
Fuel engineers prefer L/100km because it scales linearly with fuel cost. The difference between 10 and 11 L/100km is the same fuel saving as between 5 and 5.5 L/100km. With MPG, the difference between 10 and 11 MPG saves far more fuel than between 40 and 41 MPG — but people often don't realise this.
Electric Vehicle Equivalents
Electric vehicles use different efficiency units:
- Wh/km or Wh/mi: Watt-hours per distance (lower is better)
- MPGe: Miles per gallon equivalent (US) — based on the energy content of petrol
- km/kWh: Kilometres per kilowatt-hour (higher is better)
A typical efficient EV achieves around 150-200 Wh/km or 5-6 km/kWh.