A car rated 40 UK MPG doesn’t mean the same thing as a car rated 40 US MPG. A spec sheet in L/100km tells you nothing useful if you only think in miles per gallon. Buying a car listed in a unit you don’t use, or comparing two vehicles rated in different systems, leaves you guessing at numbers that don’t actually compare. A fuel unit converter solves that instantly. It turns any one of these figures into all the others at once, plus a sense of whether the number you’re looking at is actually good.
Quick Answer: 30 MPG (US) converts to roughly 7.84 L/100km, or about 12.75 km/L. Enter any single figure below and the rest convert automatically.
MPG To L/100km Conversion
Bold answer: To convert MPG to L/100km, divide 235.215 by your MPG figure. A car at 25 MPG returns 9.4 L/100km. At 32 MPG, it is 7.4 L/100km, and at 40 MPG it is 5.9 L/100km. A lower L/100km number means better fuel economy, which is the opposite of how MPG works, where higher is better.
MPG To km/L Conversion
Bold answer: To convert MPG to km/L, multiply your MPG by 0.4251. A car at 25 MPG returns 10.6 km/L. At 40 MPG it is 17.0 km/L, and at 30 MPG it is 12.8 km/L. Both MPG and km/L work the same way, a higher number means the car travels further on the same fuel.
MPG Conversion Table
| MPG | L/100km | km/L |
| 15 | 15.7 | 6.4 |
| 20 | 11.8 | 8.5 |
| 25 | 9.4 | 10.6 |
| 30 | 7.8 | 12.8 |
| 32 | 7.4 | 13.6 |
| 35 | 6.7 | 14.9 |
| 40 | 5.9 | 17.0 |
| 45 | 5.2 | 19.1 |
| 50 | 4.7 | 21.3 |
Who This Calculator Is For
Car shoppers comparing a US-market listing against a UK or European one need this to compare like-for-like. Drivers relocating between countries use it to make sense of an oddly labeled spec sheet. Anyone reading a car review that quotes L/100km, when they only think in MPG, gets an instant translation here too.
Which Unit Should You Use By Country
Fuel economy units aren’t universal. Knowing which one applies to you removes most of the guesswork before a single calculation happens. The United States runs on MPG measured against the US gallon. The United Kingdom uses MPG too, just against the larger Imperial gallon, which is exactly why the two numbers never line up directly. Canada, Australia, and most of Europe report fuel economy in L/100km instead. Japan and India both use km/L, and Pakistan commonly sees the same km/L figure, sometimes written out as kilometres per litre on local spec sheets.
Which Number Is Better
Reading these four units side by side gets confusing fast, since they don’t all move in the same direction. A higher MPG number is always better, more miles squeezed out of each gallon. A lower L/100km number is better, since it means less fuel burned to cover the same distance. A higher km/L number is better too, following the same logic as MPG just in metric terms. One rule covers all three: higher is better for MPG and km/L, lower is better for L/100km.
Conversion Formulas
Each of these four units describes the same underlying thing, fuel used over distance, just measured differently.
MPG (US) to L/100km
Divide 235.21 by your MPG figure. At 30 MPG, that works out to 7.84 L/100km.
L/100km to MPG (US)
Same formula, just flipped, 235.21 split by your L/100km figure hands back the MPG equivalent.
MPG (US) to km/L
Divide your MPG figure by 2.352. At 30 MPG, that comes to roughly 12.75 km/L.
MPG (UK) to MPG (US)
Multiply UK MPG by 0.833. A car rated 40 UK MPG converts to about 33.3 US MPG.
Why UK And US MPG Never Quite Match
This trips up more buyers than almost any other fuel figure. The UK measures fuel economy using the Imperial gallon, which holds 4.546 litres. The US gallon holds only 3.785 litres, a meaningfully smaller volume. Since MPG is miles per gallon, and the gallons themselves differ in size, the same real-world fuel efficiency produces two different MPG numbers depending on which gallon is used. A car genuinely rated 40 UK MPG only works out to about 33.3 US MPG once converted properly.
Fuel Economy Rating Guide
| Rating | MPG (US) | L/100km | km/L |
| Excellent | Over 40 | Under 5.9 | Over 17.0 |
| Good | 30–40 | 5.9–7.8 | 12.8–17.0 |
| Average | 22–30 | 7.8–10.7 | 9.3–12.8 |
| Poor | Under 22 | Over 10.7 | Under 9.3 |
Real World Vehicle Examples
A compact car typically posts around 35 MPG, an SUV closer to 24 MPG, and a pickup often settles nearer 18 MPG. Hybrids push well past all of these, commonly landing around 52 MPG, and motorcycles vary widely but often reach 65 MPG or better depending on engine size.
What Better Fuel Economy Is Worth
Moving from a 22 MPG car to a 34 MPG car doesn’t just look better on paper, it shows up in real savings. Across 15,000 miles a year, that kind of improvement commonly saves upward of $800 annually in fuel alone, depending on local fuel prices. Our fuel consumption calculator works out the exact figure once you’ve got your converted MPG in hand.
When You’ll Actually Need This Converter
A handful of situations come up again and again. Buying an imported car with a spec sheet in a unit you don’t recognize is the most common one. Reading a foreign car review, moving to a new country, or simply comparing specifications between two cars sold in different markets all call for the same conversion. Checking a manufacturer’s published figures against a government fuel label is another place this comes in handy.
Why Small Differences Matter
A conversion from 35 MPG might come out to 34.8 or 35.1 depending on rounding, and that tiny gap is nothing to worry about on its own. Where it does matter is when a rounded figure gets carried through several more calculations, since small errors compound the more math is done on top of them. This converter avoids that by keeping full decimal precision until the final number is displayed.
Metric vs Imperial Fuel Economy
The imperial system, MPG in both its US and UK forms, remains standard in the United States and holds on in the UK for everyday use, even as the country’s official testing has shifted toward metric figures. The metric system, L/100km and km/L, covers most of the rest of the world, including Canada, Australia, continental Europe, and much of Asia. Neither system is more accurate than the other, they’re just different ways of expressing the identical underlying efficiency.
Why Manufacturers Use Different Units And Test Standards
It isn’t just the unit that differs between markets, the testing method behind the number does too. The US relies on EPA test cycles built around consumer familiarity with MPG. Europe uses the WLTP cycle, which replaced the older NEDC standard, both built around a L/100km fuel consumption focus. Japan runs its own test cycle geared toward efficiency claims, and Australia follows ADR standards under a metric system by default. None of these differences make one figure more honest than another, they simply reflect different regulatory histories and consumer expectations.
Common Mistakes
Treating UK and US MPG as interchangeable leads directly to the wrong comparison between two cars. Rounding a conversion too early and carrying that rounded figure through further math compounds a small error into a larger one. Assuming a lower L/100km number means worse efficiency, when the opposite is true, trips up plenty of first-time metric readers.
Quick Reference Table
| MPG(US) | L/100km | km/L |
| 20 | 11.76 | 8.50 |
| 25 | 9.41 | 10.63 |
| 30 | 7.84 | 12.75 |
| 35 | 6.72 | 14.88 |
| 40 | 5.88 | 17.00 |
| 50 | 4.70 | 21.25 |
Related Calculators
The Fuel Consumption Calculator handles the full cost side once your units are sorted here. The MPG Calculator works out your real-world MPG from two fill-ups before you convert it. The km/L Calculator and L/100km Calculator run each metric unit as a dedicated tool. The Cost Per Mile Calculator turns whichever unit you land on into an actual running cost. The Fleet Fuel Calculator scales the same conversion across multiple vehicles at once.
Frequently Asked Questions
Methodology
Conversion factors here follow the standard relationships between gallons, litres, miles, and kilometres. They’re based on US EPA fuel economy conversion guidance and the UK’s Imperial gallon definition. All conversions use these internationally accepted constants with no intermediate rounding, so accuracy holds across all four units regardless of which one you start from. The FuelConsumptionCalc Editorial Team last reviewed this page in August 2026.