Fuel Economy Units: MPG, L/100km, and km/L Compared

Why Fuel Economy Units Differ

Fuel economy — the distance traveled per unit of fuel consumed — seems straightforward, but the world has settled on at least three different ways to express it. The United States uses miles per gallon (MPG). Europe, Australia, and Canada primarily use liters per 100 kilometers (L/100km). Japan, India, and parts of Asia use kilometers per liter (km/L).

These are not just different units on the same scale: the first and third are "efficiency" metrics (more is better), while L/100km is a "consumption" metric (less is better). This distinction matters for comparing vehicles and for understanding fuel savings.

US Miles Per Gallon (mpg US)

The US gallon is 3.78541 liters. US fuel economy is measured on EPA test cycles that simulate city, highway, and combined driving.

1 mpg (US) = 235.215 L/100km (by inversion)

This is derived from: - 1 mile = 1.60934 km - 1 US gallon = 3.78541 L - L/100km = 235.215 / mpg (US)

US Vehicle Fuel Economy Reference

CAFE (Corporate Average Fuel Economy) standards, which govern US automaker fleets, are set in mpg. For 2024, the standard for passenger cars is 49 mpg combined — manufacturers who miss this pay fines or buy credits.

UK Miles Per Gallon (mpg UK)

The UK gallon — the imperial gallon — is larger than the US gallon:

1 UK gallon = 4.54609 liters 1 US gallon = 3.78541 liters 1 UK gallon = 1.20095 US gallons

This means a car rated at 40 mpg UK achieves only 33.3 mpg US — the same vehicle, different number, because the gallon is different.

Converting UK mpg to L/100km: L/100km = 282.481 / mpg (UK)

Converting US mpg to UK mpg: mpg (UK) = mpg (US) × 1.20095

When reading car reviews that mix UK and US sources, verify which gallon is being used. A British magazine review citing "60 mpg" describes a car that an American would rate at about 50 mpg.

Liters Per 100 Kilometers (L/100km)

L/100km expresses how many liters of fuel a vehicle consumes to travel 100 kilometers. This is a consumption metric — lower is better, less fuel used.

1 L/100km = 235.215 / mpg (US) 1 mpg (US) = 235.215 / L/100km

Why L/100km Is More Mathematically Intuitive

Fuel economy stated in mpg or km/L is an efficiency metric, but it creates a mathematical trap: equal improvements in mpg do not produce equal fuel savings.

Consider two scenarios where you drive 10,000 km per year:

Scenario A: Replacing a 10 mpg (23.5 L/100km) truck with a 20 mpg (11.8 L/100km) truck - Old: 10,000 × 0.235 = 2,350 L/year - New: 10,000 × 0.118 = 1,180 L/year - Savings: 1,170 L/year

Scenario B: Replacing a 30 mpg (7.84 L/100km) car with a 40 mpg (5.88 L/100km) car - Old: 10,000 × 0.0784 = 784 L/year - New: 10,000 × 0.0588 = 588 L/year - Savings: 196 L/year

Both upgrades gain 10 mpg, but Scenario A saves 6× more fuel. L/100km makes this obvious at a glance because it is linear: 23.5 → 11.8 is a 11.7 L/100km improvement; 7.84 → 5.88 is only a 1.96 L/100km improvement.

This is why environmental economists prefer L/100km: it accurately represents actual fuel consumption and thus actual CO₂ emissions and cost.

European Fuel Economy Reference

Note: WLTP (Worldwide Harmonised Light Vehicle Test Procedure) replaced the old NEDC cycle in Europe and tends to be closer to real-world consumption than the old numbers.

Kilometers Per Liter (km/L)

Used in Japan (where it is written as カタログ燃費), South Korea, India, and parts of Southeast Asia, km/L is the efficiency equivalent of mpg — more is better.

1 km/L = 100 / L/100km 1 km/L = 2.35215 mpg (US) 1 mpg (US) = 0.425144 km/L

The Japanese JC08 test cycle (now being replaced by WLTP) gave optimistic km/L figures. A car rated at 25 km/L under JC08 typically achieved 18–20 km/L in real driving. The new WLTP figures are more realistic.

km/L Reference

EV Efficiency: kWh/100km and MPGe

Electric vehicles do not consume fuel, so fuel economy units do not directly apply. Two alternatives are in use:

kWh per 100 km (Europe/International)

This is the electric equivalent of L/100km: how many kilowatt-hours of electricity are consumed per 100 kilometers driven.

Lower is better.

MPGe (US EPA)

The US EPA converts EV efficiency to miles per gallon equivalent (MPGe) using the energy content of a gallon of gasoline: 1 US gallon of gasoline ≈ 33.7 kWh.

MPGe = 33.7 kWh/gallon ÷ kWh/mile

• Tesla Model 3 Long Range: 132 MPGe combined
• Hyundai IONIQ 6 (RWD): 140 MPGe combined

MPGe allows comparing EVs to gasoline cars using a familiar metric, though it obscures the fact that electricity and gasoline have very different production costs and well-to-wheel efficiencies.

Full Conversion Reference

Real-World Cost Comparison

Fuel cost depends on price, efficiency, and distance. At $4.00/gallon (US) and 15,000 miles/year:

The same calculation in liters: at €1.80/L and 15,000 km/year:

Explore Fuel Economy Conversions

Convert between MPG (US), MPG (UK), L/100km, and km/L at /fuel-economy/.

Summary

The US and UK both use miles per gallon but with different gallon sizes — always check which gallon is referenced. L/100km, used across Europe, Canada, and Australia, is the mathematically honest metric for fuel consumption: it scales linearly with actual fuel used and real costs. km/L, common in Japan and Asia, is intuitive but shares the mpg trap of making equal-unit improvements appear equivalent when actual savings differ greatly. For electric vehicles, kWh/100km (international) and MPGe (US) serve as the respective analogs. The core conversion factor to memorize: 235.215 = mpg (US) × L/100km.