MPG vs L/100 km: why fuel economy numbers mislead, and how to compare them

Fuel economy sounds like the simplest number on a car’s spec sheet. In practice it’s one of the most misread. The same car can be quoted as 30, 36 or 7.8 depending on where you are, and even within one country, the way the number is expressed leads people to the wrong conclusion about which upgrade saves the most fuel. This guide sorts out the units, shows how to convert them exactly, and explains the “MPG illusion” that trips up almost everyone.

Two gallons, one name

The US gallon is defined as 231 cubic inches, exactly 3.785411784 litres. The imperial gallon, used in the UK, is exactly 4.54609 litres. The imperial gallon is about 20% larger, so a car travels further on one, and its MPG figure is higher.

• 30 MPG (US) = 36.0 MPG (imperial)
• 30 MPG (imperial) = 25.0 MPG (US)

This matters any time you read a review, a forum post or a spec sheet from another country. A UK figure of 45 MPG and a US figure of 45 MPG don’t describe equally efficient cars. It also catches out British drivers in a quieter way: fuel is sold in litres in the UK, but car economy is still usually quoted in imperial MPG, so working out a trip cost means converting units anyway.

Economy versus consumption

MPG and km/L tell you how far you go on a unit of fuel. They’re economy measures, and bigger is better. Litres per 100 kilometres tells you how much fuel you use to cover a fixed distance. It’s a consumption measure, and smaller is better. Canada’s EnerGuide labels show consumption in L/100 km alongside economy in imperial miles per gallon, and most of Europe and Australia also quote L/100 km.

The two families are reciprocals, which makes the conversions simple once you have the constants:

L/100 km = 235.215 ÷ MPG (US) L/100 km = 282.481 ÷ MPG (imperial) L/100 km = 100 ÷ km/L MPG (imperial) = MPG (US) × 1.20095

The constant 235.215 is 100 × 3.785411784 ÷ 1.609344: litres per US gallon, kilometres per mile, and the 100 in “per 100 km”.

Notice how the MPG columns bunch up at the bottom of the table. Going from 15 to 12 L/100 km is a step of 3 litres; going from 5 to 4 is a step of 1. In MPG terms, the first looks like a gain of about 4 MPG and the second like a gain of about 12 MPG. The MPG numbers make the smaller saving look bigger.

The MPG illusion

In 2008, Richard Larrick and Jack Soll of Duke University’s Fuqua School of Business published a study in Science showing that people systematically misjudge fuel savings when efficiency is expressed in MPG. Asked which saves more fuel over 10,000 miles, most people pick an improvement from 34 to 50 MPG over one from 18 to 28 MPG. It’s the other way round.

• 18 → 28 MPG: 556 gallons becomes 357 gallons per 10,000 miles, a saving of 198 gallons.
• 34 → 50 MPG: 294 gallons becomes 200 gallons, a saving of 94 gallons.

The reason is the reciprocal. Fuel used is distance divided by MPG, so each extra MPG saves less fuel than the one before. Soll gave a neat example: moving a vehicle from 10 to 20 MPG saves 5 gallons per 100 miles, while moving from 25 to 50 MPG saves only 2. The authors suggested expressing efficiency as gallons per 10,000 miles. US regulators moved in that direction: since the 2013 model year, US fuel economy labels have shown a fuel consumption rate in gallons per 100 miles alongside MPG.

The practical lesson is counter-intuitive but useful. If a household has two vehicles and can improve one, improving the least efficient one usually saves the most fuel. And when comparing a car you have with one you’re considering, work out fuel per year rather than eyeballing the MPG difference.

Measuring your own figure

Official ratings come from standard laboratory test cycles, which are useful for comparing cars but rarely match any one driver’s experience. Your own figure is easy to measure with the fill-up method: fill the tank to the click, reset the trip meter, drive normally, fill to the click again, and divide distance by fuel. Over several tanks, add up all the distance and all the fuel before dividing, rather than averaging each tank’s MPG; otherwise a short tank counts as much as a long one. The MPG calculator does this for up to six fill-ups and shows the result in all four units.

Turning economy into money

Cost follows consumption directly: fuel used × price per unit. That’s why consumption units make budgeting easier. At 7.8 L/100 km and a price per litre, a 500 km trip uses 39 litres; multiply by the price and you’re done. With MPG you first divide distance by MPG to get gallons, and you have to be sure the gallon in the MPG figure matches the gallon in the price.

The fuel cost calculator handles every combination of distance, economy and price unit, and converts everything to litres and kilometres internally so the unit you choose never changes the answer. If you’re comparing a fuel car with an electric one, the EV charging cost calculator does the same for kWh.

A quick conversion checklist

1. Find out which gallon a figure uses. UK sources usually mean imperial; US sources mean US gallons.
2. Convert to L/100 km (or gallons per 100 miles) before comparing savings.
3. Compare fuel used over a realistic distance, such as a year of your driving, not MPG differences.
4. Use your own measured figure for cost estimates where you can.

Summary

Fuel economy numbers aren’t hard, but they’re easy to misread. Pin down the gallon, remember that MPG and L/100 km are reciprocals, and judge savings by fuel used rather than by the size of the MPG change. Do that and the numbers will point you to the choices that genuinely save the most fuel and money.

Sources

• Duke Today (2008), on Larrick and Soll’s “The MPG Illusion”, Science.
• US DOT: EPA and DOT unveil next generation of fuel economy labels (gallons per 100 miles, model year 2013).
• Natural Resources Canada: EnerGuide for vehicles (L/100 km and imperial mpg).
• NIST Handbook 44, Appendix C (US gallon) and UK Weights and Measures Act 1985, Schedule 1 (imperial gallon).