MPG vs L/100km: Fuel Efficiency Explained
Miles per gallon (MPG) and litres per 100 km (L/100km) both measure fuel efficiency but express it in opposite ways — and that creates a hidden distortion when comparing vehicles. Understanding the difference helps you make better decisions when choosing a car, planning a trip with the fuel cost calculator, or evaluating whether an upgrade is worth it.
The Key Difference
MPG measures how far you travel per unit of fuel — bigger is better. L/100km measures how much fuel you use per unit of distance — smaller is better. They are reciprocals of each other, but not linear ones: a 5 MPG improvement at 15 MPG saves far more fuel than a 5 MPG improvement at 45 MPG.
MPG to L/100km Conversion Table
| MPG (US) | L/100km | Fuel type |
|---|---|---|
| 15 MPG | 15.7 L/100km | Large truck / old SUV |
| 20 MPG | 11.8 L/100km | Mid-size SUV / van |
| 25 MPG | 9.4 L/100km | Average car |
| 30 MPG | 7.8 L/100km | Efficient sedan / small crossover |
| 35 MPG | 6.7 L/100km | Hybrid-like efficiency |
| 40 MPG | 5.9 L/100km | Hybrid (non-plug-in) |
| 50 MPG | 4.7 L/100km | Toyota Prius / efficient hybrid |
Conversion formula: L/100km = 235.2 ÷ MPG (using US gallons). For UK MPG: L/100km = 282.5 ÷ MPG (UK gallon = 4.546 litres vs US gallon = 3.785 litres).
Why L/100km Shows Savings More Accurately
Consider improving a car from 15 to 20 MPG (a 5 MPG gain) versus 40 to 50 MPG (also a 10 MPG gain). Which saves more fuel on a 1,000-mile trip?
- 15 → 20 MPG: 66.7 gallons → 50 gallons = 16.7 gallons saved
- 40 → 50 MPG: 25 gallons → 20 gallons = 5 gallons saved
The 15→20 MPG upgrade saves more than 3× the fuel despite being the same nominal improvement. In L/100km terms this is obvious: 15.7 → 11.8 L/100km (a 3.9 L drop) versus 5.9 → 4.7 L/100km (a 1.2 L drop). The linear scale of L/100km makes the real-world impact immediately visible.
Which Metric to Use When
Use MPG for
- Comparing vehicles in the US market (all EPA ratings use MPG)
- Estimating range from a tank (tank size ÷ MPG = max miles)
- Conversations with other US drivers — it is the universal reference
Use L/100km for
- Comparing vehicles in Europe, Australia, or Canada
- Calculating actual fuel consumed for a trip (easier arithmetic)
- Evaluating whether an efficiency upgrade is worth the cost
- Comparing vehicles with very different efficiencies (the linear scale is more honest)
City vs Highway MPG: Which to Use?
The EPA city figure covers stop-and-go urban driving; highway covers sustained motorway speeds. Combined is a weighted average (55% city, 45% highway). For fuel cost estimates:
- Road trips: use highway MPG
- Daily commute (mixed): use combined MPG
- City-only driving: use city MPG
Real-world efficiency typically runs 10–20% below EPA estimates due to speed, load, climate, and driving style. Add a 10% buffer to any cost estimate from the fuel cost calculator if you want a conservative budget figure.
Calculate Fuel Cost in Any Unit System
US MPG, UK MPG/litre, or metric L/100km — instant results for any trip distance. Free, no signup.
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