Fuel Efficiency Units
Fuel efficiency is expressed as distance per volume (MPG — higher is better) or volume per distance (L/100km — lower is better). Converting between them is essential for comparing vehicles across markets.
Conversions
L/100km → US MPG: 235.21 / L/100km
US MPG → L/100km: 235.21 / MPG
US MPG → UK MPG: US MPG × 1.201 (UK gallon = 4.546L)
UK MPG → L/100km: 282.48 / UK MPG
Examples:
7 L/100km = 235.21/7 = 33.6 US MPG = 40.3 UK MPG
30 US MPG = 235.21/30 = 7.84 L/100kmTrip Fuel Cost
Fuel used = Distance / (100/L_per_100km)
= Distance × L/100km / 100
Cost = Fuel used × Price per litre
500km trip at 7L/100km, £1.50/L:
Fuel = 500 × 7/100 = 35 L
Cost = 35 × 1.50 = £52.50Typical Fuel Efficiency by Vehicle Type
- Small car (petrol): 5-7 L/100km (34-47 US MPG)
- SUV/crossover: 8-11 L/100km (21-29 US MPG)
- Hybrid: 4-6 L/100km (39-59 US MPG)
- EV: 15-20 kWh/100km (not mpg — different metric)
Calculate fuel efficiency: Free Fuel Efficiency Calculator
Fuel Efficiency Quick-Reference Conversion Table
| MPG (US) | MPG (UK) | L/100km | km/L |
|---|---|---|---|
| 15 | 18.0 | 15.7 | 6.4 |
| 25 | 30.0 | 9.4 | 10.6 |
| 30 | 36.0 | 7.8 | 12.7 |
| 40 | 48.0 | 5.9 | 17.0 |
| 50 | 60.1 | 4.7 | 21.3 |
| 60 | 72.1 | 3.9 | 25.5 |
US gallon = 3.785 L; UK gallon = 4.546 L. L/100km = 235.2 / MPG(US). km/L = 0.4251 × MPG(US).
How Fuel Efficiency Works
Fuel efficiency measures how far a vehicle travels per unit of fuel. MPG (miles per gallon) is the US and UK standard; L/100km (litres per 100 kilometres) is used in most other countries; km/L is common in Asia and Latin America. These are reciprocals scaled by distance: improving from 20 to 25 MPG saves more fuel per kilometre than improving from 40 to 50 MPG — a counter-intuitive result because efficiency improvements have diminishing returns at higher values.
Real-world fuel efficiency depends on speed (drag increases with v²), tyre pressure, air conditioning load, load weight, driving style (aggressive acceleration and braking), road gradient, fuel type, and engine temperature. Highway MPG exceeds city MPG for most combustion vehicles because idling in traffic wastes fuel with zero distance gained. Hybrid vehicles reverse this — regenerative braking makes city driving more efficient than highway cruising.
Common Mistakes
- Comparing MPG and L/100km linearly: Halving L/100km (from 10 to 5) doubles fuel efficiency. But a 5 MPG improvement from 15 to 20 MPG saves far more fuel per mile than the same 5 MPG from 45 to 50 MPG. Always compare fuel consumed per distance when evaluating savings.
- US vs. UK gallons: UK (Imperial) gallon = 4.546 L; US gallon = 3.785 L. A car rated at 40 MPG in the UK achieves only 33.3 MPG on the US scale — a 20% difference. Comparing specs across markets requires conversion.
- Official vs. real-world figures: EPA/WLTP test cycle figures are consistently optimistic. Real-world MPG is typically 15–25% lower than official figures for most petrol/diesel vehicles; electric vehicle range shows similar discrepancy at highway speeds in cold weather.
Frequently Asked Questions
Aerodynamic drag increases with the square of speed; engine efficiency peaks at moderate loads. For most cars, fuel efficiency peaks around 80–90 km/h (50–55 mph). At 110 km/h vs. 90 km/h, fuel consumption increases by roughly 25%. Driving at 130 km/h vs. 90 km/h increases consumption by ~50%. These numbers vary by vehicle aerodynamics — SUVs and trucks degrade more steeply with speed than aerodynamic sedans.
Each 10% drop in tyre pressure below the recommended level reduces fuel economy by approximately 0.5–1% (EPA estimate). Severely under-inflated tyres (20%+ below spec) can reduce MPG by 3–5%. Beyond efficiency, under-inflation increases tyre wear, reduces handling precision, and raises blowout risk at highway speeds. Check tyre pressure monthly — tyres lose ~1 PSI per month naturally and ~1 PSI per 10°F temperature drop.
Hypermiling techniques: (1) Maintain steady speed using cruise control on flat roads. (2) Anticipate stops and coast to decelerate rather than braking. (3) Accelerate smoothly — 0-to-60 in 15 seconds uses far less fuel than 8 seconds. (4) Reduce highway speed. (5) Remove roof racks and cargo when not in use. (6) Park in shade to reduce cabin cooling load. Skilled hypermilers regularly exceed EPA estimates by 20–40% without modifying the vehicle.