mpg vs L/100km: Converting Fuel Economy and Avoiding the mpg Illusion

Miles per gallon and litres per 100 km both describe how much fuel a car uses, but they're built upside down from each other. One measures distance per unit of fuel; the other measures fuel per unit of distance. That inversion makes conversions awkward and, more importantly, makes mpg figures easy to misread when you're deciding which car upgrade actually saves the most fuel. This guide covers the conversion maths, the US versus UK gallon trap, and why the "mpg illusion" catches almost everyone.

Two ways to measure the same thing

• Miles per gallon (mpg) is how far you go on one gallon, so higher is better. The US uses it, and so does the UK with a bigger gallon.
• Litres per 100 kilometres (L/100km) is how much fuel you burn to cover 100 km, so lower is better. It's used across most of Europe, Australia, Canada and many other countries.
• Kilometres per litre (km/L) is distance per litre, so higher is better. This is what most Indian buyers see as "mileage" or kmpl.

mpg and km/L are the same kind of measure with different units. L/100km is the inverse kind. That's where all the confusion starts.

The conversion formulas

One US gallon is 3.785411784 litres and one mile is 1.609344 km. Combining those gives a single constant you can divide by:

Because it's a division, not a multiplication, the same formula works in both directions. A car that does 30 mpg (US) uses 235.215 ÷ 30 = 7.84 L/100km. A car rated at 6 L/100km does 235.215 ÷ 6 = 39.2 mpg (US). An Indian hatchback rated at 20 km/L uses 100 ÷ 20 = 5 L/100km.

US gallons versus UK gallons

A UK (imperial) gallon is 4.54609 litres, about 20% bigger than a US gallon. So the same car gets a higher mpg number in the UK. To convert between them, multiply US mpg by 1.20095, or divide UK mpg by the same figure. For UK mpg to L/100km, the constant is 282.481 instead of 235.215.

This matters when you read reviews or forums from another country. A British article praising a car's "50 mpg" is describing about 41.6 mpg in US terms. Mixing them up overstates or understates fuel use by a fifth.

The mpg illusion

Here's a question. Which saves more fuel over 10,000 miles: replacing a 10 mpg truck with a 20 mpg one, or replacing a 30 mpg car with a 50 mpg one? Most people pick the second. The jump from 30 to 50 looks huge. Do the arithmetic, though:

• Going from 10 to 20 mpg cuts fuel from 10,000 ÷ 10 = 1,000 gallons to 10,000 ÷ 20 = 500 gallons. That saves 500 gallons.
• Going from 30 to 50 mpg cuts fuel from 10,000 ÷ 30 = 333 gallons to 10,000 ÷ 50 = 200 gallons. That saves 133 gallons.

The truck upgrade saves almost four times as much fuel. Researchers Richard Larrick and Jack Soll described this in a 2008 paper in Science and called it the "mpg illusion": people tend to treat mpg as if fuel savings rise in a straight line with it, when they actually shrink as mpg rises. Each extra mpg is worth less than the one before.

In L/100km the same comparison is obvious. 23.5 → 11.8 L/100km is a drop of 11.8 litres per 100 km. 7.8 → 4.7 is a drop of 3.1 litres. Because L/100km measures fuel directly, savings scale in a straight line and the bigger number really is the bigger saving. That's the main reason many economists prefer fuel-per-distance units, and why US window stickers now also show gallons per 100 miles.

Turning efficiency into money

Fuel cost per year is easy once you're in fuel-per-distance units:

Take someone driving 15,000 km a year in India, comparing a car that does 15 km/L with one that does 18 km/L. In L/100km that's 6.67 versus 5.56. Annual fuel is 1,000 litres versus 833 litres, a difference of about 167 litres. At an assumed ₹100 per litre, that's roughly ₹16,700 a year. If the more efficient car costs ₹1,00,000 more, it takes about six years of fuel savings to cover the gap, before counting interest on the extra money or any difference in resale value.

The Fuel Cost Calculator does this for a specific trip if you'd rather not do it by hand.

Shortcuts for mental conversion

You don't always have a converter handy. A few relationships make rough conversions easy:

• Halving one doubles the other. 10 L/100km is about 23.5 mpg (US); 5 L/100km is about 47. The same is true going from mpg to L/100km.
• Around 24 mpg (US) is about 10 L/100km. It's a useful anchor for mid-sized petrol cars and SUVs. 235 ÷ 24 = 9.8.
• For km/L, divide 100. 12.5 km/L is 8 L/100km. 25 km/L is 4 L/100km.
• Every 1 L/100km you save is 1 litre per 100 km driven. At 15,000 km a year that's 150 litres, whatever car you start from. No mpg figure gives you a rule that simple.

That last point is the practical reason to think in L/100km when you compare cars, even if the brochure gives you mpg or km/L. Convert both figures, subtract, and multiply by your yearly distance. The answer is in litres, which turns straight into money.

Why your real figures don't match the brochure

Official fuel economy comes from lab tests run on a set driving cycle. Different regions use different cycles: the EPA tests in the US, WLTP in Europe, and the MIDC-based procedure used for Indian ARAI ratings. These cycles differ in speed, acceleration and idle time, so the same car gets different official numbers in different markets. Converting a European WLTP figure to US mpg gives you a US-unit number, but not an EPA rating.

Real-world driving adds things the tests don't fully capture: air conditioning, heavy traffic, short cold-start trips, roof boxes, under-inflated tyres and aggressive acceleration. Most drivers see worse figures than the label. The gap varies so much by driver and conditions that the best number to use is your own, from a full-tank-to-full-tank log.

1. Fill the tank completely and reset the trip meter.
2. Drive normally until the tank is at least half empty.
3. Fill it completely again and note the litres added and kilometres driven.
4. km ÷ litres gives km/L; litres ÷ km × 100 gives L/100km.

Electric cars use different units

EVs don't burn fuel, so their efficiency is shown as energy per distance, usually kWh/100km or Wh/km. In the US you'll also see MPGe, which treats 33.7 kWh of electricity as equivalent to one gallon of petrol. MPGe is useful for comparing an EV with a petrol car on energy use, but it says nothing about cost, since electricity and petrol are priced very differently. For running costs, multiply kWh/100km by your price per kWh.

Limitations of the converter

• It converts units, not test cycles. The Fuel Economy Converter turns 40 mpg (US) into 5.88 L/100km exactly. It can't tell you what a car rated 40 mpg on the EPA cycle would score on WLTP.
• Zero has no real answer. Because L/100km and mpg are inverses, 0 L/100km would mean infinite mpg. The converter shows 0 in that case rather than infinity, so treat a zero result as "not applicable".
• Check which gallon you have. The converter offers both US and UK mpg. Picking the wrong one silently gives a figure that's 20% off.
• Rounding adds up. Converting back and forth several times with rounded numbers drifts. Convert from the original figure each time.