Understanding Fuel Efficiency: MPG, km/L, and L/100km Explained
Three Systems, One Concept
Fuel efficiency measures how far a vehicle can travel on a given amount of fuel. Different regions use different units, which makes comparing cars across markets surprisingly tricky:
- Miles per gallon (MPG) — Used in the US and UK
- Kilometres per litre (km/L) — Used in India, Japan, and parts of Asia and Latin America
- Litres per 100 kilometres (L/100km) — Used in Europe, Canada, Australia, and China
The first two measure distance per unit of fuel (higher = better). The third measures fuel per unit of distance (lower = better). This inverse relationship is the biggest source of confusion.
MPG: Miles Per Gallon
Used in the US and UK, but with a critical difference: US and UK gallons are not the same size.
| System | Gallon Size | Effect on MPG |
|---|---|---|
| US MPG | 3.785 litres | Lower number |
| UK (Imperial) MPG | 4.546 litres | Higher number (~20% more) |
This means a car rated at 30 MPG (US) would be rated at about 36 MPG (UK) — same car, same efficiency, different numbers. When comparing MPG ratings across US and UK sources, always check which gallon is being used.
km/L: Kilometres Per Litre
Popular in India, Japan, South Korea, and parts of Latin America. It's arguably the most intuitive unit: "How many kilometres can I drive on one litre of fuel?"
A car getting 15 km/L can travel 15 km on one litre. If your tank holds 45 litres, your range is 15 × 45 = 675 km.
India's ARAI (Automotive Research Association of India) certifies fuel efficiency in km/L. However, real-world figures are typically 15–25% lower than ARAI ratings due to differences in test conditions vs actual driving.
L/100km: The Inverse Unit
Used across Europe, Canada, Australia, and China. Instead of asking "how far can I go on one unit of fuel," it asks "how much fuel do I need to go 100 km?"
This inverse framing confuses many people at first, but it has a mathematical advantage: fuel savings are linear. Going from 10 L/100km to 8 L/100km always saves exactly 2 litres per 100 km, regardless of starting efficiency. With MPG, going from 15 to 17 MPG saves more fuel than going from 35 to 37 MPG — a phenomenon called the "MPG illusion."
The MPG Illusion
Consider upgrading two vehicles, each driving 10,000 miles per year:
- SUV: 14 MPG → 17 MPG — saves 126 gallons/year
- Sedan: 35 MPG → 45 MPG — saves 63 gallons/year
The SUV's 3 MPG improvement saves twice as much fuel as the sedan's 10 MPG improvement! In L/100km, this would be obvious: the SUV went from 16.8 to 13.8 (saving 3.0), while the sedan went from 6.7 to 5.2 (saving 1.5). Linear savings, no illusion.
Conversion Formulas
| From → To | Formula |
|---|---|
| km/L → L/100km | L/100km = 100 ÷ km/L |
| L/100km → km/L | km/L = 100 ÷ L/100km |
| km/L → MPG (US) | MPG = km/L × 2.35215 |
| MPG (US) → km/L | km/L = MPG ÷ 2.35215 |
| MPG (US) → L/100km | L/100km = 235.215 ÷ MPG |
| L/100km → MPG (US) | MPG = 235.215 ÷ L/100km |
| MPG (US) → MPG (UK) | MPG (UK) = MPG (US) × 1.20095 |
Fuel Efficiency Benchmarks
| Vehicle Type | km/L | MPG (US) | L/100km |
|---|---|---|---|
| Large SUV / truck | 6–9 | 14–21 | 11–17 |
| Mid-size sedan | 12–16 | 28–38 | 6–8 |
| Compact car | 16–22 | 38–52 | 4.5–6 |
| Hybrid | 20–30 | 47–70 | 3.3–5 |
| Diesel sedan | 18–25 | 42–59 | 4–5.5 |
| Motorcycle | 25–50 | 59–118 | 2–4 |
Electric Vehicles: Different Metrics
Electric vehicles don't use fuel in the traditional sense, so they have their own efficiency metrics:
- kWh/100km — Used in Europe. A typical EV uses 15–20 kWh/100km. Lower = better.
- MPGe (miles per gallon equivalent) — US EPA rating. Based on the energy content of one gallon of gasoline (33.7 kWh). A Tesla Model 3 rates ~130 MPGe.
- Wh/mi or Wh/km — Watts per mile/km. Used by EV enthusiasts for real-world tracking.
- km/kWh — Direct analogue to km/L. A typical EV gets 5–7 km/kWh.
10 Tips to Improve Fuel Efficiency
- Maintain steady speed. Use cruise control on highways. Constant acceleration and braking wastes fuel.
- Check tyre pressure monthly. Under-inflated tyres increase rolling resistance. Proper inflation improves efficiency by 2–3%.
- Remove excess weight. Every extra 45 kg (~100 lb) reduces efficiency by about 1%.
- Don't idle. Turn off the engine if you'll be stopped for more than 60 seconds. Modern engines use less fuel restarting than idling.
- Use the recommended fuel grade. Premium fuel in a regular-grade engine wastes money without improving efficiency.
- Drive in the highest gear possible. Lower RPMs generally mean lower fuel consumption.
- Reduce aerodynamic drag. Remove roof racks when not in use. Close windows at highway speeds.
- Keep up with maintenance. Clean air filters, fresh spark plugs, and proper oil all contribute to efficiency.
- Plan routes. Shorter distances and fewer stops mean less fuel wasted in traffic and acceleration.
- Accelerate gently. Aggressive acceleration can increase fuel consumption by 15–30%. Pretend there's an egg under the pedal.