MPG, L/100km or km/L? Reading Any Car's Fuel Economy Without Getting Fooled

By the Super Simple Digital Tools Team · Updated June 2026 · Converters

Few specifications are quoted in as many incompatible ways as fuel economy. A car sold in the United States wears a miles-per-gallon sticker, the same model in France advertises litres per 100 kilometres, and in Japan or India it is rated in kilometres per litre. None of these are wrong, but they cannot be compared side by side until they share a unit. That is the whole job of a fuel consumption converter, and understanding the logic behind it stops you from making a poor buying or budgeting decision based on a number that only looked good.

The first thing to internalise is direction. Miles per gallon and kilometres per litre both answer the question 'how far can I go on a set amount of fuel?', so bigger is better. Litres per 100 kilometres flips the question to 'how much fuel does it take to go a set distance?', so smaller is better. This inversion is why you divide rather than multiply when converting, and it is also why people misread brochures: an 8 L/100km car is thirstier than a 6 L/100km car, even though 8 is the larger number.

The second trap is the gallon. The United States and the United Kingdom both say 'MPG', but they do not mean the same gallon. An Imperial gallon holds about 4.546 litres against the US gallon's 3.785, roughly a fifth more fuel. The practical result is that 1 US MPG equals about 1.201 UK MPG. A used import advertised at '50 MPG' from a British seller is really around 42 US MPG, a gap big enough to change your fuel-budget maths over a year of driving.

Once direction and gallon type are settled, the conversions are simple division. US MPG to L/100km is 235.215 divided by the figure; UK MPG to L/100km is 282.481 divided by the figure; km/L to L/100km is 100 divided by the figure. Those constants are not arbitrary: 235.215 is the litres in a US gallon multiplied by the number of miles in 100 kilometres. Run a few values and the pattern becomes intuitive: 40 US MPG is about 5.9 L/100km, and 5 L/100km is about 47 US MPG.

Finally, treat any converted number as exact arithmetic on an inexact input. The conversion itself is precise, but a manufacturer's lab-cycle rating, your trip computer's running average, and a real tank-to-tank measurement can all disagree because driving style, load, terrain, and weather move the needle. Use the converter to make figures comparable, then judge them against how and where you actually drive rather than the showroom claim.

Try it now: Open the Fuel Consumption Converter →

Quick tips

Always confirm whether an MPG figure is US or Imperial before converting; the 20% gallon difference is the single biggest source of error.
When comparing cars across markets, convert everything to one unit first, ideally L/100km, since it makes fuel cost per trip easy to estimate.
Remember that lower L/100km is better but higher MPG and km/L are better, so don't compare raw numbers across systems.
For quick mental checks, memorise that 235 divided by US MPG gives L/100km, and 100 divided by km/L gives L/100km.

The Fuel Consumption Converter is free to use as often as you like — no signup required.