Rent a car in Germany and the brochure might say the model uses 5.2 liters per 100 kilometers. Read a US review of a similar engine and you see 32 mpg combined. Open a British magazine and the same hardware might be listed near 40 mpg. None of those numbers is wrong. They are just written in different dialects of fuel economy, and the gallon hiding behind two of them is not even the same size.
This guide walks through the four labels you see most often: US miles per gallon, UK miles per gallon, liters per 100 km, and kilometers per liter. Once you know how they connect, you can sanity-check a trip budget, compare imports fairly, and avoid the classic mistake of mixing US and UK mpg when you shop online.
Four Ways to Describe the Same Tank of Fuel
US mpg is miles traveled per US liquid gallon (about 3.785 liters). EPA window stickers and American automotive press use this.
UK mpg is miles per imperial gallon (about 4.546 liters). British magazines, dealer sheets, and some Commonwealth material still quote it.
Liters per 100 km (often written L/100 km) tells you how many liters you burn to drive 100 kilometers. It is standard across most of Europe and many other metric markets. Lower is better, which catches people who grew up with mpg and instinctively reach for bigger numbers.
Kilometers per liter (km/L) is common in Japan and parts of Asia. Higher is better, like mpg, but the volume in the denominator is one liter, not a gallon.
Remember the gallons: The imperial gallon is roughly twenty percent larger than the US gallon. That alone inflates UK mpg compared with US mpg for the same real-world consumption. Always check which gallon a source uses before you brag or complain about efficiency.
US MPG and Liters per 100 km
To move between US mpg and L/100 km you only need one constant for everyday work: 235.21. It comes from the exact definitions of the US gallon and the international mile.
US mpg to L/100 km: divide 235.21 by the mpg figure.
Example: 30 US mpg gives 235.21 ÷ 30 ≈ 7.84 L/100 km.
L/100 km to US mpg: divide 235.21 by the L/100 km figure.
Example: 8.0 L/100 km gives 235.21 ÷ 8.0 ≈ 29.4 US mpg.
If you prefer kilometers per liter first, note that US mpg = (km/L) × 2.352 (2.35214583 if you want full precision). Going the other way, km/L = US mpg ÷ 2.352.
UK Imperial MPG
British mpg uses the larger gallon, so the shortcut divisor changes. A handy constant is 282.48.
UK mpg to L/100 km: divide 282.48 by the UK mpg value.
Example: 45 UK mpg becomes 282.48 ÷ 45 ≈ 6.28 L/100 km.
L/100 km to UK mpg: divide 282.48 by L/100 km.
To compare US and UK mpg directly for the same car, convert both to L/100 km (or km/L) and compare those. Quoting both raw mpg numbers side by side without labeling the gallon type is how arguments start.
What You See on the Ground
Rental desks, peer-to-peer car apps, and long forum threads rarely agree, even when everyone uses liters. One figure might come from a manufacturer catalog under ideal lab conditions. Another might be a trip computer average after a week of city driving. A third could be an owner’s hand calculation at the pump. The unit is honest; the driving pattern behind it usually is not.
Short trips in winter routinely push L/100 km upward because cold engines and cabin heat demand more fuel for the first few kilometers. Motorway cruising does the opposite. If you translate a brochure’s 5.0 L/100 km into 47 US mpg and then wonder why your real holiday average is closer to 7.5 L/100 km, the gap is often traffic and temperature, not a broken converter.
Diesel and gasoline vehicles share the same labels, but people interpret them differently. A mid-size European diesel that shows 5.5 L/100 km on a long run sounds ordinary to locals. An American driver hearing “thirty-two mpg” for a similar segment might expect more, until they remember the US figure was probably EPA combined while the European one was extra-urban cruising. Converting both numbers into one system clears up most of that confusion.
Japanese listings favor km/L. If you see 18 km/L on a domestic site, that is roughly 42 US mpg using the 2.352 factor. Dealers in export markets sometimes round aggressively, so take a second pass with your own division when money is on the line.
Label check: If a spec sheet says “mpg” without “US” or “imperial,” look at the company’s mailing address. US brands default to US gallons. British publications default to imperial. When in doubt, convert to L/100 km and compare there.
Why Test Cycles Still Matter More Than the Unit
Even perfect arithmetic cannot make a European WLTP figure directly equivalent to a US EPA combined rating. Drive cycles, ambient temperature rules, and rounding differ. Treat conversions as a ballpark for shopping and trip planning, not as proof that one regulator is kinder than another.
If you are importing a used vehicle or reading global press, convert the numbers, then read which standard produced them. The unit is only half the story.
Road-trip check: If you know your car averages 7.5 L/100 km and fuel costs 1.85 euros per liter, every 100 km needs about 13.88 euros in fuel. Convert distance to miles if that is how you think about legs of the journey, then multiply. The hard part is rarely the liter itself; it is remembering whether your mental model started in US mpg or UK mpg when you first learned the car.
| US mpg | L/100 km (approx.) | km/L (approx.) |
|---|---|---|
| 20 | 11.76 | 8.50 |
| 25 | 9.41 | 10.63 |
| 30 | 7.84 | 12.75 |
| 35 | 6.72 | 14.88 |
| 40 | 5.88 | 17.01 |
| 45 | 5.23 | 19.14 |
| 50 | 4.70 | 21.26 |
Electric Labels: MPGe in Brief
Battery electric vehicles add MPGe, miles per gasoline-equivalent energy. It helps compare electrics to combustion cars on one scale, but daily life with an EV still runs on kilowatt-hours per 100 miles or per 100 km depending on the market. If you are cross-shopping, convert the electric figures using the same care you use for mpg: note the country, the test, and whether the sticker shows consumption or range.
Convert Fuel, Distance, and Volume
Use Convert Units HQ for mpg-related math alongside liters, gallons, miles, and kilometers.
Open the ConverterQuick Reference: Same Economy, Different Numbers
| L/100 km | US mpg (approx.) | UK mpg (approx.) |
|---|---|---|
| 4.0 | 58.8 | 70.6 |
| 5.0 | 47.0 | 56.5 |
| 6.0 | 39.2 | 47.1 |
| 7.0 | 33.6 | 40.4 |
| 8.0 | 29.4 | 35.3 |
| 9.0 | 26.1 | 31.4 |
| 10.0 | 23.5 | 28.2 |
Fuel economy is one of the few places where a bigger printed number does not always mean a thriftier car. Pick one intermediate language (most people settle on L/100 km or US mpg), translate everything into it, and label your gallons. That habit saves more money than memorizing extra trivia ever will. When the math needs to be exact, Convert Units HQ is there to handle the factors so you can focus on driving.