Car specs are published in four completely different systems depending on where the vehicle was built: the United States uses miles per US gallon (MPG), the United Kingdom still quotes miles per imperial gallon, Europe and most of the rest of the world use liters per 100 km (L/100km), and Japanese and Korean brochures often list kilometers per liter (km/L). A 25 MPG car in Los Angeles, a 30 MPG car in London, a 9.4 L/100km car in Berlin, and a 10.6 km/L car in Tokyo are all describing the exact same real-world fuel thirst — yet the numbers look nothing alike.
This fuel efficiency converter takes any one of those four figures and instantly rewrites it in the other three, so you can compare a US window-sticker rating against a European spec sheet, shop a UK import, or sanity-check a used-car listing without doing the algebra in your head. Because MPG and L/100km move in opposite directions — higher MPG means lower fuel use, while lower L/100km means lower fuel use — the conversion is a reciprocal, not a simple multiply, which is why online tables are easy to misread.
How to Use
- Type the Fuel Economy Value exactly as it appears on the spec sheet, ad, or window sticker. If the number is 25, enter 25 — do not add units.
- Pick the From Unit that matches that source: US MPG, UK MPG, L/100km, or km per liter. Getting this step wrong is the single most common reason conversions come out backwards.
- Pick the To Unit you want the answer in. If you are comparing cars side by side, set this to whichever rating system you already think in — most US shoppers pick US MPG, most European shoppers pick L/100km.
- Hit convert. The result card shows the converted figure in bold, plus the equivalent value in all four units at once so you never have to flip back and forth.
- Use the four-equivalent row to cross-check listings: if a UK ad quotes 40 mpg and the converter tells you that is 33.3 US mpg and 7.06 L/100km, you now have an apples-to-apples number to compare against a US-market rival.
A quick rule of thumb: a 1 MPG improvement on a thirsty old V8 saves far more fuel than the same 1 MPG gain on a modern hybrid, because MPG is a reciprocal. That is one reason governments publish L/100km for fuel-economy rules — it is a linear measure, so halving it literally halves fuel use.
The Conversion Math
The engine normalizes every input to L/100km, then normalizes out to the target unit, using the internationally agreed conversion constants:
- 1 US liquid gallon = 3.785411784 liters
- 1 UK imperial gallon = 4.54609 liters
- 1 statute mile = 1.609344 kilometers
From those:
- L/100km = 235.214583 ÷ US MPG
- L/100km = 282.480936 ÷ UK MPG
- L/100km = 100 ÷ km per liter
Going the other way: US MPG = 235.214583 ÷ L/100km, UK MPG = 282.480936 ÷ L/100km, and km per liter = 100 ÷ L/100km. The US and UK constants differ by about 20% because an imperial gallon is roughly 20% larger than a US gallon — that gap is why a car advertised as "40 mpg" in London is only about 33 mpg by US EPA standards.
Worked example: 235.214583 ÷ 25 US MPG gives 9.41 L/100km. Flip it around: 235.214583 ÷ 8.0 L/100km gives 29.4 US MPG. Notice that 8.0 L/100km is the European fuel-economy threshold most buyers recognize as "efficient" — it maps to about 29 US MPG, not to 40, which is the kind of mistake shoppers make when they eyeball a spec sheet.
Examples
Example 1 — Comparing two cars on the dealer lot. A US-market sedan is advertised at 30 MPG combined, while a European import on the same lot claims 7.0 L/100km. Enter 30 in US MPG → L/100km: you get 7.84 L/100km. The European car at 7.0 L/100km is genuinely more efficient — about 11% less fuel per kilometer. At 12,000 miles per year and $3.50 per US gallon, the US car burns about $1,400 in fuel; the European car, converted back, is roughly 33.6 MPG and burns about $1,250 — a $150 per year gap.
Example 2 — Reading a UK import ad. A UK listing quotes a diesel at 40 mpg (imperial). A US shopper who assumes that is US MPG will expect 40 US MPG. Converting 40 UK MPG to US MPG gives only 33.3 US MPG, and to L/100km gives 7.06 L/100km. The real difference between 33.3 and 40 MPG over 15,000 miles at $3.50/gal is about $264 per year (450 vs 375 gallons) — enough to change the asking-price judgment.
Example 3 — Japanese spec sheet. A hybrid brochure lists 22.0 km/L. Converting to US MPG gives 51.7 MPG, and to L/100km gives 4.55 L/100km. That is the kind of figure that looks impossible to a US shopper until you remember km/L rewards small displacement stop-and-run cycles heavily; real-world US EPA testing will land closer to 45–48 MPG. Use the converter to translate the number, then apply a 10–15% real-world discount for the test-cycle gap.
FAQ
What is the difference between US MPG and UK MPG?
A US gallon is 3.785 liters and a UK (imperial) gallon is 4.546 liters — about 20% larger. The same car therefore travels further on one UK gallon than on one US gallon, so UK MPG numbers look roughly 20% higher than US MPG numbers for the identical vehicle. Use the From Unit selector to pick UK MPG, and the converter applies the 282.480936 constant instead of the 235.214583 one.
Why do US cars use MPG while Europe uses L/100km?
MPG measures distance per volume, so it is intuitive for drivers ("how far can I go on a tank?"). L/100km measures volume per distance, so it is linear: cutting from 10 to 5 L/100km literally halves fuel use. Because fuel-tax policy and emissions rules reward linear efficiency, Europe, China, Australia and most of Asia publish L/100km; the US, UK, and a handful of other markets still use MPG.
Is 25 MPG good or bad?
It depends on the class. A modern compact sedan at 25 MPG combined is below average; a full-size pickup at 25 MPG is excellent. Converted, 25 US MPG equals 9.41 L/100km and about 30 UK MPG. Use the four-equivalent row to compare against a spec sheet you trust, rather than judging the raw number in isolation.
How do I convert L/100km back to MPG?
Use the converter with From Unit = L/100km and To Unit = US MPG (or UK MPG). The math is MPG = 235.214583 ÷ L/100km for US gallons. As a quick sanity check, 8 L/100km is about 29 US MPG, 6 L/100km is about 39 US MPG, and 5 L/100km is about 47 US MPG.
Does the converter work for electric vehicles?
EVs are rated in miles per kWh (MPK) or kWh per 100 km, not in fuel-economy volume units, so this converter does not take a kilowatt-hour figure. It is designed for petrol, diesel, and hybrid ICE vehicles. If you are comparing a hybrid's petrol-only MPG to an EV's MPK, use a dedicated EV-cost-per-mile tool and compare dollar figures rather than efficiency units.
Why is a 1 MPG gain on an old V8 worth more than a 1 MPG gain on a hybrid?
Because MPG is a reciprocal. Going from 15 to 16 MPG saves about 42 gallons per 10,000 miles; going from 45 to 46 MPG saves only about 5 gallons over the same distance. L/100km hides this effect because it is linear — dropping from 15.7 to 14.8 L/100km and dropping from 5.2 to 5.0 L/100km both mean roughly the same liter savings. If you are shopping a used car, convert every candidate to L/100km before ranking them.
Disclaimer
This fuel efficiency converter is provided for informational purposes only and does not constitute financial, automotive, or regulatory advice. Conversions use standard international constants (US gallon 3.785411784 L, UK gallon 4.54609 L, mile 1.609344 km); real-world fuel economy depends on driving style, terrain, load, temperature, fuel blend, and test cycle. EPA, WLTP, NEDC, and JCO8 ratings are not directly interchangeable even after unit conversion. Always confirm published figures on the manufacturer's official spec sheet before purchasing or importing a vehicle.