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Kalcufy

MPG to km/L Converter

Convert US mpg to km/L, L/100 km and imperial mpg — one of these three scales runs backwards.

Conversion table

Fuel economyIn km per litre
10.42514 km/L
20.85029 km/L
31.2754 km/L
41.7006 km/L
52.1257 km/L
104.2514 km/L
156.3771 km/L
208.5029 km/L
2510.6286 km/L
3012.7543 km/L
4017.0057 km/L
5021.2571 km/L
6025.5086 km/L
7029.76 km/L
7531.8857 km/L

🚙 Landmarks on all three scales

  • 30 mpg (US)12.75 km/L
  • 30 mpg (US)7.84 L/100 km
  • 1 US gallon3.785 L
  • 1 imperial gallon4.546 L

Reading fuel figures without being fooled

  • Higher is better in mpg and km/L; lower is better in L/100 km. The third scale runs the opposite way, so a "better" number can mean a worse car.
  • British mpg figures are about 20% higher than American ones for the same car, because the imperial gallon is 4.546 litres against 3.785.
  • To convert US mpg to L/100 km, divide 235.215 by the mpg. For imperial mpg, divide 282.481 instead.
  • Fuel saved is not proportional to mpg gained. Replacing a 15 mpg vehicle with a 20 mpg one saves more fuel than replacing a 35 with a 50.

Three scales, one of which is inverted

Miles per gallon and kilometres per litre both measure distance per unit of fuel, so a bigger number means a more efficient car. Litres per 100 kilometres measures the opposite — fuel per unit of distance — so a smaller number is better. That inversion is why the conversion is a division rather than a multiplication: litres per 100 km equals 235.215 divided by US mpg, and the constant comes straight from the definitions, being 100 times 3.785411784 litres divided by 1.609344 kilometres. Europe standardised on L/100 km precisely because fuel consumption, not distance, is what you are budgeting.

The MPG illusion, and why the EPA added a second figure

Because distance sits on top of the fraction, equal improvements in mpg do not produce equal fuel savings. Over 10,000 miles, moving from 10 to 15 mpg saves 333 gallons, while moving from 30 to 40 saves only 83 — even though the second jump looks larger. Larrick and Soll published this in Science in 2008 and named it the MPG illusion, showing that people consistently rank the wrong upgrade as the bigger saving. The practical fix is to think in fuel per distance, which is what L/100 km does natively and why the American window sticker now also prints gallons per 100 miles.

The two gallons, and other traps

  • A US gallon is 3.785411784 litres; an imperial gallon is 4.54609 litres, about 20% larger.
  • The same car is quoted at 30 mpg in the United States and 36 mpg in the United Kingdom without changing anything.
  • Canada abandoned mpg for L/100 km in the 1970s, so a Canadian brochure and an American one for the same model look nothing alike.
  • US EPA figures come from standard test cycles and are separated into city, highway and combined; brochure figures elsewhere use WLTP.
  • Electric cars are rated in kWh per 100 km or in MPGe, which converts electricity into an energy-equivalent gallon.
  • Trip computers usually reset with the tank; a lifetime average is the more honest number.

What each figure looks like in practice

  • A large SUV: 18–22 mpg US, which is 7.7–9.4 km/L or 10.7–13.1 L/100 km.
  • A mid-size petrol saloon: 28–33 mpg US, 11.9–14.0 km/L, 7.1–8.4 L/100 km.
  • A small diesel: 50–60 mpg US, 21–25.5 km/L, 3.9–4.7 L/100 km.
  • A full hybrid: 50–58 mpg US, around 21–24.7 km/L or 4.1–4.7 L/100 km.
  • City driving is typically 20–30% worse than highway for petrol cars, and often better for hybrids, which recover braking energy.
  • A 10 km/h drop in motorway speed usually gains more real economy than any driving-style advice.

Worked examples

Across all three scales

A sticker says 30 mpg

  1. 30 × 0.425144 = 12.75 km/L
  2. 235.215 ÷ 30 = 7.84 L/100 km
  3. In imperial mpg: 30 × 1.20095 = 36.0

30 mpg US = 12.75 km/L = 7.84 L/100 km

A European car quotes 6 L/100 km

  1. 235.215 ÷ 6 = 39.2 mpg US
  2. 100 ÷ 6 = 16.67 km/L
  3. In imperial mpg: 282.481 ÷ 6 = 47.1

6 L/100 km = 39.2 mpg US = 16.67 km/L

Frequently Asked Questions

How do I convert mpg to km/L?

Multiply US mpg by 0.425144. So 30 mpg is 12.75 km/L and 40 mpg is 17.0 km/L. The factor is simply 1.609344 kilometres per mile divided by 3.785411784 litres per US gallon.

How do I convert mpg to L/100 km?

Divide 235.215 by the US mpg figure. So 30 mpg becomes 7.84 L/100 km and 50 mpg becomes 4.70. It is a division rather than a multiplication because L/100 km is an inverted scale: more fuel per distance means a worse car.

Why is UK mpg higher than US mpg for the same car?

Because the imperial gallon holds 4.54609 litres and the US gallon only 3.785411784 — about 20% more fuel in the British gallon, so the car covers 20% more miles on it. A car rated 30 mpg in America is 36 mpg in Britain with no mechanical difference whatsoever.

Is L/100 km better than mpg?

It is less misleading, because it puts fuel on top. Fuel cost scales directly with litres per 100 km, so halving the figure halves the fuel bill. With mpg the relationship is inverted, which is what produces the MPG illusion — people underestimate the savings from improving a thirsty vehicle.

Which saves more fuel: 15 to 20 mpg, or 35 to 50 mpg?

The first, by a wide margin. Over 10,000 miles, 15 to 20 mpg saves 167 gallons; 35 to 50 mpg saves 143. This is the MPG illusion in one line: because distance is on top of the fraction, small gains at the thirsty end are worth more than large gains at the efficient end.

Does my trip computer show the same thing as the sticker?

Not usually. Official figures come from standardised test cycles under controlled conditions, while a trip computer measures your driving, in your traffic and your weather. Expect a real-world figure 10 to 20% worse than the label, and use the lifetime average rather than a single tank.

Sources