What is fuel economy?
Fuel economy (or fuel efficiency) measures how far a vehicle travels per unit of fuel, or how much fuel it burns per distance. The two families are mirror images: MPG, km/L, and miles/L express distance per volume (higher is better), while L/100km expresses volume per distance (lower is better). Because L/100km is the reciprocal of the others, you cannot convert it with a simple "multiply by a factor" — the math is inverted. Our fuel economy converter handles this correctly, so you get trustworthy numbers every time.
US vs UK gallons — a hidden trap
Two "MPG" values exist. The US gallon is 3.78541 liters; the UK (imperial) gallon is 4.54609 liters. A car rated 40 MPG in Britain is only about 33.3 MPG by US measure — a 20% gap that surprises travelers. Always confirm which gallon a spec uses. Our tool keeps both MPG (US) and MPG (UK) separate and labeled.
The base-unit method (with an inverse)
We use liters per 100 km (L/100km) as the internal base because it is the dominant standard in Europe, Canada, Australia, and much of Asia. To convert any unit into the base: MPG(US) → base = 378.541 ÷ MPG; MPG(UK) → base = 454.609 ÷ MPG; km/L → base = 100 ÷ km/L; miles/L → base = 100 ÷ (miles/L × 1.60934). To leave the base: MPG(US) = 378.541 ÷ base, and so on. The reciprocal step is what makes L/100km consistent with the distance-per-volume units.
Key conversion formulas
- L/100km = 378.541 ÷ MPG(US)
- L/100km = 454.609 ÷ MPG(UK)
- L/100km = 100 ÷ km/L
- km/L = 100 ÷ L/100km
- MPG(US) = 2.35215 × km/L
- MPG(UK) = 1.20095 × MPG(US)
Reading the numbers
A "good" MPG is high; a "good" L/100km is low. Roughly: 30 MPG(US) ≈ 7.84 L/100km ≈ 12.75 km/L. A thrifty compact at 50 MPG(US) ≈ 4.71 L/100km. A gas-hungry V8 at 15 MPG(US) ≈ 15.7 L/100km. One liter of fuel taking you 20 km is the same as 5 L/100km. Keeping both mental models helps when comparing a US EPA sticker with a European spec sheet.
Real-world examples
Suppose your rental in Europe is rated 5.5 L/100km and you want the US equivalent: 378.541 ÷ 5.5 ≈ 42.8 MPG(US). A US truck at 18 MPG(US) becomes 13.06 L/100km — not great. A hybrid at 4.2 L/100km = 23.8 km/L = 56 MPG(US). If you think in miles per liter, 1 MPG(US) = 0.425 km/L, so 30 MPG(US) ≈ 12.75 miles/L. These swaps are exactly what the converter does in the background.
Common mistakes
The most common error is treating US and UK MPG as equal — they are not. The second is inverting L/100km wrongly: people multiply instead of divide, turning "better" into "worse." Remember the direction flips between the two families. The third is comparing a US EPA number (often optimistic) with a European WLTP number (often closer to real driving) without accounting for test-cycle differences — the unit conversion is exact, but the underlying ratings are not identical in the real world.
Frequently asked questions
Base unit for fuel economy?
We use L/100km internally; every unit converts to it then to the target, including the inverse L/100km.
Why is L/100km different from MPG?
MPG is distance/volume (higher better); L/100km is volume/distance (lower better). They are reciprocals.
US vs UK MPG?
1 UK MPG = 1.20095 US MPG (UK gallon is larger). Not interchangeable.
30 MPG(US) to L/100km?
L/100km = 378.541 ÷ MPG(US), so 30 ≈ 12.62 L/100km.
Is km/L the same as MPG?
No: 1 MPG(US) = 0.425 km/L; 1 km/L = 2.35215 MPG(US).