Last updated: 2026-05-18

Four Units, One Measurement

Fuel economy — how far a vehicle travels on a given amount of fuel — is expressed in four common units worldwide, and they are not all equivalent:

Unit	Used In	Better =
L/100km	Europe, Korea, Middle East	Lower
MPG (US)	United States, Canada	Higher
MPG (UK)	United Kingdom	Higher
km/L	Japan, Brazil, Southeast Asia	Higher

This fragmentation creates genuine confusion when comparing a Japanese car's stated efficiency (km/L) with a European figure (L/100km) or an American rating (MPG US).

The Reciprocal Relationship

L/100km and km/L are exact reciprocals: one measures fuel per distance, the other measures distance per fuel. To convert between them:

$$\text{km/L} = \frac{100}{\text{L/100km}}$$ $$\text{L/100km} = \frac{100}{\text{km/L}}$$

A car that gets 10 L/100km also gets exactly 10 km/L — the numbers happen to be the same in this example, but they measure the same efficiency from opposite perspectives.

MPG (US and UK) also share this reciprocal relationship with L/100km, using the gallon size as the connecting factor.

US Gallon vs UK Gallon

A key source of confusion is that "MPG" means different things in the US and UK:

Gallon	Volume	Conversion Factor
US gallon	3.785 L	1 L/100km = 235.215 MPG(US)
UK (Imperial) gallon	4.546 L	1 L/100km = 282.481 MPG(UK)

Because a UK gallon holds about 20% more fuel than a US gallon, the same vehicle will always have a numerically higher UK MPG figure than US MPG. A car rated at 30 MPG (US) is approximately 36 MPG (UK) — the car has not become more efficient, only the unit of measurement changed.

As a rough guide, multiply US MPG by 1.201 to get UK MPG.

Conversion Formulas

All conversions pass through L/100km as a common base:

$$\text{MPG (US)} = \frac{235.215}{\text{L/100km}}$$ $$\text{MPG (UK)} = \frac{282.481}{\text{L/100km}}$$ $$\text{km/L} = \frac{100}{\text{L/100km}}$$

The constants 235.215 and 282.481 come from combining the mile-to-kilometre ratio (1 mile = 1.60934 km) with each gallon's litre equivalent.

Efficiency Direction by Unit

Unit	More Fuel-Efficient =
L/100km	Lower number (less fuel burned per 100 km)
MPG (US or UK)	Higher number (more miles per gallon)
km/L	Higher number (more km per litre)

The direction of efficiency varies by unit. A figure of 5 means nothing without the unit: 5 L/100km is excellent efficiency, while 5 MPG is an extremely poor rating (typical for large trucks towing heavy loads).

Typical Values for Passenger Cars

Vehicle Type	L/100km	MPG (US)	km/L
Fuel-efficient compact	4–6	39–59	17–25
Average sedan	7–10	24–34	10–14
SUV / crossover	10–14	17–24	7–10
Large truck or van	14–20	12–17	5–7
Hybrid (e.g. Toyota Prius)	~4.5	~52	~22

Electric Vehicles

Electric vehicles do not use litres or gallons, so fuel economy converters do not apply directly. Instead, EVs are rated in:

• kWh/100km (Europe, similar logic to L/100km — lower is better)
• MPGe (US — miles per gallon equivalent, an energy-equivalent conversion)
• km/kWh (Japan, similar to km/L — higher is better)

Comparing a petrol car to an EV requires an energy-equivalence calculation rather than a simple unit conversion.

Frequently Asked Questions (FAQ)

How do I convert MPG to L/100km?

Divide 235.215 by the MPG (US) value to get L/100km. For example, 235.215 ÷ 30 MPG (US) ≈ 7.84 L/100km. For UK MPG, divide 282.481 by the value instead. The division is necessary because the units are reciprocals of each other — higher MPG corresponds to lower L/100km.

Why is UK MPG always higher than US MPG for the same car?

A UK (Imperial) gallon is larger than a US gallon: 4.546 L vs. 3.785 L. So the same car travels more miles before running out of a UK gallon than a US gallon. The conversion factor is approximately 1.201 — multiply US MPG by 1.201 to get UK MPG. A car rated 30 MPG (US) is approximately 36 MPG (UK).

Why does a lower L/100km number mean better fuel economy?

L/100km measures how much fuel is consumed per 100 km of travel. A lower value means less fuel used for the same distance — more efficient. This is the inverse of MPG and km/L, where higher values indicate better efficiency.

How do I convert km/L to MPG?

To convert km/L to US MPG, multiply by 2.3521 (equivalently, divide 235.215 by (100 ÷ km/L)). For example, 15 km/L × 2.3521 ≈ 35.3 MPG (US). Alternatively, convert km/L to L/100km first by dividing 100 by the km/L value, then apply the MPG formula.