What is the difference between MPG and L/100km?
MPG counts distance per unit of fuel: how many miles one gallon buys. L/100km counts fuel per unit of distance: how many litres a fixed 100 km trip costs. Higher MPG is better; lower L/100km is better. They are reciprocals of each other, linked by a constant, so 235.214583 divided by an MPG (US) figure gives L/100km and the same division in reverse gives MPG back.
The two arose in different places for practical reasons. MPG belongs to countries that measured road distance in miles and sold fuel by the gallon, which meant Britain and its colonies, then the United States. L/100km belongs to the metric world, where the natural question became how much fuel a journey of round-number length would use. Japan and India, metric but with their own habits, settled on kilometres per litre, which is MPG in metric clothing.
Why does the same improvement look different on each scale?
Because a reciprocal scale is not linear. Going from 10 to 20 mpg saves 11.76 litres on every 100 km, since 23.52 L/100km becomes 11.76. Going from 40 to 50 mpg, the same 10 mpg gain, saves 1.18 litres, since 5.88 becomes 4.70. The first improvement saves ten times as much fuel as the second, and MPG presents them as identical.
L/100km has the opposite property. Each litre saved per 100 km is worth the same amount of fuel and money whatever the starting point. Going from 12 to 11 L/100km and from 5 to 4 L/100km both save one litre per 100 km. On the MPG scale those look like a gain of 1.8 mpg and a gain of 11.8 mpg respectively, wildly different for the same saving.
This is sometimes called the MPG illusion, after a 2008 paper in Science by Larrick and Soll that showed people consistently misjudge fuel savings when figures are presented in MPG. Asked which saves more fuel, replacing a 10 mpg car with a 20 mpg one or replacing a 25 mpg car with a 50 mpg one, most people pick the second. The first saves nearly twice as much.
Which unit gives the honest answer when comparing cars?
L/100km, or any fuel-per-distance unit, because fuel is what you pay for and distance is what you need. If you drive 15,000 km a year, a car at 8 L/100km uses 1,200 litres and a car at 6 uses 900. The saving of 300 litres reads directly off the figures. In MPG the same cars are 29.4 and 39.2, and the saving is not obvious without a calculator.
The practical rule for anyone stuck with MPG figures is to convert them before comparing. Turn both cars into L/100km with the MPG to L/100km converter, or into gallons per 100 miles, which is the same idea in imperial units and which the EPA has printed on US window stickers since 2013 for exactly this reason. The comparison then becomes a subtraction.
None of this makes MPG wrong. As a description of one car it is perfectly clear: 40 mpg means 40 miles on a gallon. It is only when comparing or thinking about improvements that the reciprocal scale misleads, and that is most of what fuel economy figures are used for.
Why do the US and UK still use MPG?
Distance. Both countries sign their roads in miles and their speed limits in miles per hour, so a fuel economy figure in miles feels native even where the fuel is bought in litres, as it is in Britain. Changing the road signs would be an enormous, visible undertaking; leaving the mpg figure in place costs nothing. Habit does the rest.
The US has a second reason. It still sells fuel by the gallon, so mpg is built from two units drivers see every day. Britain does not, and the result is a figure in miles per a gallon nobody has bought since the early 1990s, using the imperial gallon of 4.54609 litres. The US vs UK gallon guide explains how that came about.
Canada, Australia, New Zealand, Ireland and South Africa all used mpg with imperial gallons and all switched to L/100km when they metricated their roads. The switch followed the road signs, not the pumps, which supports the idea that distance units drive the choice.
Where does km/L fit in?
Kilometres per litre is metric MPG: distance per fuel, higher is better, and it carries the same reciprocal illusion when comparing cars. Japan, India, Brazil, Indonesia, Thailand, Pakistan and the Gulf states use it, which by population makes it the most widely spoken fuel economy unit in the world. Its numbers are smaller than mpg because the litre is smaller than the gallon and the kilometre shorter than the mile.
Converting km/L to L/100km is a division: 100 divided by the km/L figure. 20 km/L is 5 L/100km. Converting to mpg uses the km/L to MPG page, which offers both gallons. If you think in km/L and want to compare two cars honestly, the L/100km conversion is the one to make first.
Which should you use?
For describing one car to someone in your own country, use the local unit; anything else invites confusion. For comparing two cars, or for working out what an improvement is worth, convert to L/100km or gallons per 100 miles first, then subtract. For your own records, pick one unit and keep it, since a log that switches units is a log you will misread later.
Whichever you choose, write down the gallon when the unit is mpg. A number without a gallon is a number nobody else can use, and a table of them is a table that will be misread in exactly the way this guide has been describing.