MPG vs L/100km: How Fuel Economy Units Compare

Two different ways of measuring the same thing

L/100km and MPG both describe how efficiently a vehicle uses fuel, but they measure it in opposite directions. L/100km asks how much fuel is consumed to cover a fixed distance, so it is a consumption rate -- lower numbers mean less fuel burned and therefore better economy. MPG asks how far a vehicle travels on a fixed amount of fuel, so it is a distance-per-fuel rate -- higher numbers mean more distance covered per gallon and therefore better economy.

Because L/100km and MPG measure the relationship between distance and fuel from opposite ends, they are mathematical reciprocals of one another rather than the same scale read in a different unit. This is the reason a calculator or spreadsheet cannot simply multiply an MPG figure by a fixed factor to get L/100km -- the correct conversion requires dividing a constant by the value, not multiplying it.

Why the two units are not linearly convertible

Because MPG is the reciprocal of a consumption rate, equal-sized improvements in MPG do not represent equal-sized fuel savings, and the reverse is also true: equal-sized improvements in L/100km always represent the same fuel savings per 100 km driven, regardless of the starting point. Increasing fuel economy from 10 L/100km to 8 L/100km saves exactly the same 2 liters per 100 km as improving from 6 L/100km to 4 L/100km, but the same two changes look very different when expressed in MPG.

This reciprocal relationship is sometimes called the 'MPG illusion' in the consumer psychology literature: because MPG scales change unevenly relative to actual fuel consumption, people tend to underestimate the fuel savings from replacing a low-MPG vehicle and overestimate the savings from replacing an already-efficient high-MPG vehicle. Larrick and Soll (2008), published in Science, demonstrated this effect experimentally and argued that a linear consumption metric such as L/100km (or gallons per 100 miles) communicates fuel-saving comparisons more accurately than MPG.

The exact gallon and mile constants

Converting L/100km to MPG requires two exact definitions: the size of a gallon and the length of a mile. A US gallon is defined as exactly 3.785411784 liters, while the UK imperial gallon is defined as exactly 4.54609 liters -- about 20% larger than the US gallon. Both countries use the same mile, defined as exactly 1.609344 kilometers, so the mile is not the source of the difference between US and UK MPG figures; the gallon is.

Combining these constants gives a direct conversion: MPG (US) is approximately 235.215 divided by the L/100km figure, and MPG (UK) is approximately 282.481 divided by the L/100km figure. These constants are themselves derived from the gallon and mile definitions (100 x liters-per-gallon, divided by kilometers-per-mile), so they are exact to the precision of the underlying unit definitions, not independently rounded approximations.

Worked example: converting 7.31 L/100km

Consider a vehicle that uses 38 liters of fuel to travel 520 km, a real-world fuel economy of (38 ÷ 520) × 100 ≈ 7.31 L/100km, equivalent to 520 ÷ 38 ≈ 13.68 km/L. Converting to miles per US gallon: divide km/L by 1.609344 to get miles per liter (13.68 ÷ 1.609344 ≈ 8.50), then multiply by the US gallon's 3.785411784 liters to get approximately 32.2 miles per US gallon.

Converting the same real-world fuel economy to the UK gallon uses the identical miles-per-liter figure (8.50) multiplied by the UK gallon's 4.54609 liters instead, giving approximately 38.7 miles per UK gallon. Both MPG figures describe exactly the same car using exactly the same amount of fuel over exactly the same distance -- the only thing that changed is which country's gallon was used as the unit of fuel volume.

The reciprocal relationship in numbers

The table below applies the direct conversion constants to a range of L/100km values, showing the corresponding MPG (US) and MPG (UK) figures. Reading down the L/100km column in equal 2-unit steps, the MPG columns do not fall in equal steps -- they fall by progressively smaller amounts, which is the visible signature of a reciprocal (inverse) relationship rather than a linear one.

Why the same car reports different MPG figures in the US and UK

A vehicle sold in both the United States and the United Kingdom with identical real-world fuel economy will report a higher MPG figure in UK sources than in US sources, purely because of the gallon size difference. Since the UK imperial gallon holds about 20% more fuel than the US gallon, a UK gallon carries a vehicle about 20% further, producing a numerically higher miles-per-gallon figure for the exact same underlying fuel efficiency.

This is a common source of confusion when comparing fuel economy figures quoted in American publications against figures quoted in British or Commonwealth publications for what appears to be the same vehicle: the difference is not a discrepancy in the car's real performance, but a difference in which gallon definition was used to express the same real-world result.

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Источники

1. National Institute of Standards and Technology (NIST) -- standard unit conversion factors, including the US gallon (3.785411784 L) and the mile (1.609344 km). nist.gov.
2. UK Weights and Measures Act 1985 -- statutory definition of the imperial gallon (4.54609 L).
3. US Environmental Protection Agency -- Fuel Economy and MPG testing/label information. fueleconomy.gov.
4. Larrick RP, Soll JB. "The MPG Illusion." Science, 2008;320(5883):1593-1594.