MPG, km/L and L/100km: The World's Three Fuel Economy Systems

Three Systems, One Question

Every fuel economy figure in the world is trying to answer the same question: how far does a car travel on a given amount of fuel? The answer just gets expressed in three different ways depending on where you live. The UK uses miles per gallon (UK MPG), the United States uses miles per gallon (US MPG), much of Asia uses kilometres per litre (km/L), and most of Europe uses litres per 100 kilometres (L/100km). They all describe the same physical reality, but they don't all describe it equally well.

Where Each System Is Used

• UK MPG (Imperial gallon = 4.546 L): United Kingdom. Still the dominant figure in consumer journalism and dealer adverts despite the country going metric for fuel sales decades ago.
• US MPG (US gallon = 3.785 L): United States, parts of Latin America. Posted on the EPA fuel economy label of every new car.
• km/L: Japan, India, much of Southeast Asia, parts of South America. Intuitive — bigger is better, and the units match the road signs.
• L/100km: The European Union, Australia, New Zealand, South Africa, Canada (alongside MPG). The official measure under WLTP.

The Conversion Formulas

Memorise two constants and you can convert anything to anything:

• UK MPG ↔ L/100km: divide 282.5 by either to get the other
• US MPG ↔ L/100km: divide 235.2 by either to get the other
• km/L ↔ L/100km: divide 100 by either to get the other
• UK MPG → US MPG: multiply by 0.833 (US gallons are smaller)
• km/L → UK MPG: multiply by 2.825

Conversion Table for Common Figures

Why L/100km Is Mathematically the Most Useful

L/100km expresses fuel use as a function of distance, which is the way fuel actually behaves: a car burns a roughly fixed amount of fuel per kilometre of road covered. That makes the number linear with fuel use. If a car uses 8 L/100km and another uses 4 L/100km, the second car genuinely uses half as much fuel. Multiply distance by L/100km and divide by 100 — you have litres, ready to multiply by price.

MPG inverts this relationship. It measures distance per unit fuel rather than fuel per unit distance, which means the number gets less sensitive at the high end and more sensitive at the low end. That sounds like statistical pedantry until you realise it changes how people make decisions about which car to buy.

The MPG Illusion

In a 2008 paper published in Science, behavioural researchers Richard Larrick and Jack Soll demonstrated what they called the MPG illusion. People asked to choose between fuel economy upgrades systematically misjudge which one saves more fuel.

Consider two upgrades over 10,000 miles:

1. Replacing a 12 MPG vehicle with an 18 MPG vehicle
2. Replacing a 28 MPG vehicle with a 50 MPG vehicle

Most people pick the second — 22 extra MPG sounds far more impressive than 6. In fact, the first saves 278 gallons over 10,000 miles, while the second saves only 157 gallons. Going from 12 to 18 MPG is roughly twice as valuable as going from 28 to 50 MPG, despite looking less dramatic.

L/100km exposes the truth immediately. The first upgrade goes from 23.5 to 15.7 L/100km — a saving of 7.8 L per 100 km. The second goes from 10.1 to 5.6 L/100km — a saving of 4.5 L per 100 km. The bigger litre saving wins. This is exactly why the EU mandated L/100km on car labels.

Practical Interpretation

If you're comparing two cars, convert both figures to L/100km (or US MPG, equivalently inverse) before deciding. The percentage MPG difference is not the percentage fuel-cost difference — only the L/100km difference is.

If you're tracking your own car's economy, L/100km is also easier to log: divide the litres you put in by the kilometres since the last fill, multiply by 100. With MPG you need to divide miles by gallons, which is fine but doesn't telegraph your actual fuel spend the way L/100km does.

And if you're shopping for a second car in a household that already owns one inefficient vehicle, the MPG illusion suggests the biggest fuel saving usually comes from upgrading the worst car, not buying a hyper-efficient new one to sit alongside it.

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