Fuel Economy Converter
US and imperial mpg differ by 20%.
US and imperial mpg differ by 20%. Fuel economy across miles per gallon, kilometres per litre and litres per 100 km, with a running cost.
Fuel consumption
7.84 L/100 km
30 US mpg · 36.03 imperial mpg
Miles per gallon and litres per 100 km run in opposite directions and are reciprocals, so no fixed offset converts between them. The larger trap is that a US gallon is 3.785 litres and an imperial gallon 4.546 — a 20% difference, which makes an unlabelled mpg figure ambiguous rather than merely imprecise, and makes cross-Atlantic vehicle comparisons systematically wrong.
How the Fuel Economy Converter works
Fuel economy across miles per gallon, kilometres per litre and litres per 100 km, with a running cost. The two gallons differ by 20%, which makes an unlabelled mpg figure ambiguous rather than merely imprecise.
Also known as: mpg to l/100km converter · us mpg to uk mpg · kilometres per litre converter · fuel consumption conversion
The mpg illusion
Equal improvements in miles per gallon do not save equal fuel. Going from 10 to 20 mpg saves five gallons per hundred miles; going from 40 to 50 saves half a gallon — a tenfold difference for the same nominal gain.
The reason is that mpg is a reciprocal measure. Fuel used is proportional to gallons per mile, not miles per gallon, so improvements at the inefficient end are worth far more than the numbers suggest.
That is why litres per 100 km is the better unit for decisions. It is directly proportional to fuel consumed, so equal reductions save equal fuel. Countries using it make efficiency comparisons that mpg users systematically misjudge.
Two gallons, twenty percent apart
A US gallon is 3.785 litres; an imperial gallon is 4.546. The same vehicle reads about 20% higher in imperial mpg than in US mpg, purely because the gallon is larger.
An unlabelled mpg figure is therefore ambiguous rather than merely imprecise. Comparing a US review's figure against a UK one without converting produces a systematically wrong conclusion.
The divergence dates to 1824, when Britain redefined the imperial gallon as the volume of ten pounds of water while the US kept the older English wine gallon. Two centuries later it is still catching people out.
Why official figures overstate
Official economy comes from standardised test cycles run on a rolling road under controlled conditions. The point is comparability between vehicles, not representativeness of any particular driver.
Newer standards narrowed the gap without closing it — a 10 to 20% real-world shortfall remains normal. Test conditions exclude the accessories, weather, traffic and driving style that dominate real consumption.
Speed matters more than most drivers assume. Aerodynamic drag rises with the square of speed, so dropping from 80 to 70 mph typically saves a noticeable double-digit percentage on a long motorway run — more than most other measures combined.
Where to go next
The Fuel Economy Converter question rarely arrives on its own. These are the ones that usually come with it:
- Speed Converter — A knot is one nautical mile an hour, exactly.
- Energy Converter — A food calorie is a thousand physics calories.
- Length Converter — An inch has been exactly 25.4 mm since 1959.
- Ring Size Calculator — Finger measurement to US, UK, EU and Japanese sizes.
Frequently asked questions
How do I convert mpg to litres per 100 km?
They are reciprocals, so there is no additive conversion. Divide 235.2 by US mpg, or 282.5 by imperial mpg, to get litres per 100 km.
Why do US and UK mpg figures differ?
Because a US gallon is 3.785 litres and an imperial gallon is 4.546 — about 20% larger. The same car reads about 20% higher in imperial mpg.
Which measure is better, mpg or l/100 km?
Litres per 100 km is more useful for comparing savings, because it is proportional to fuel used. Improving from 20 to 25 mpg saves far more fuel than improving from 40 to 45, which mpg obscures.
What is the mpg illusion?
The fact that equal mpg improvements do not save equal fuel. Going from 10 to 20 mpg saves five gallons per hundred miles; going from 40 to 50 saves half a gallon. It is why consumption-based units are preferred.
Why is my real economy worse than the official figure?
Official figures come from standardised test cycles that do not match real driving. Newer standards narrowed the gap without closing it, and a 10 to 20% shortfall remains normal.
How do I compare an electric vehicle?
Use kilowatt-hours per 100 miles or per 100 km, and compare against electricity price. Converting an EV to an mpg-equivalent requires assumptions about generation efficiency that make the number hard to interpret.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/fuel-economy-converter" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Fuel Economy Converter"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/fuel-economy-converter" style="color:#64748b">Fuel Economy Converter</a> from The Calc Library</p>The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.
Related calculators
Speed Converter
A knot is one nautical mile an hour, exactly.
OpenEnergy Converter
A food calorie is a thousand physics calories.
OpenLength Converter
An inch has been exactly 25.4 mm since 1959.
OpenRing Size Calculator
Finger measurement to US, UK, EU and Japanese sizes.
Open