Fuel Economy and Mileage Conversions — km/L, L/100km and mpg
Fuel economy is reported in three different, genuinely incompatible units depending on where you are: India and most of the world use kilometres per litre (km/L) or litres per 100 kilometres (L/100km), the United States uses miles per US gallon (mpg), and the United Kingdom uses miles per imperial gallon — a different-sized gallon from the American one. None of these are simple decimal multiples of each other, and mixing them up is an easy, embarrassing mistake in a unit-conversion or dimensional-analysis question. This article gives the exact conversion factors and walks through real numbers so you can do the conversion confidently in either direction.
km/L (from US mpg) = mpg × 1.609344 ÷ 3.785412 = mpg × 0.425144
km/L (from UK/imperial mpg) = mpg × 1.609344 ÷ 4.546092 = mpg × 0.354006
What each quantity means: km/L and mpg are both "distance per unit of fuel" — a bigger number means better economy. L/100km is the opposite: "fuel per unit of distance" — a smaller number means better economy. This inversion is exactly where most conversion mistakes happen. The two gallon sizes differ because the US gallon is defined as 3.785412 litres while the UK (imperial) gallon is 4.546092 litres — about 20% larger — so a car rated "40 mpg" in a UK review and "40 mpg" in a US review are not achieving the same fuel economy at all.
L/100km = 100 ÷ 18 = 5.5555… ≈ 5.56 L/100km.
Check by working backwards: 5.56 L/100km means 5.56 L takes you 100 km, so 1 L takes you 100 ÷ 5.56 = 17.99 ≈ 18 km/L. Matches.
km/L = 32 × 0.425144 = 13.60 km/L (rounded to 2 d.p.).
Cross-check from first principles: 32 miles = 32 × 1.609344 = 51.499 km. 1 US gallon = 3.785412 L. So km/L = 51.499 ÷ 3.785412 = 13.605 km/L. Matches within rounding.
US: 45 × 0.425144 = 19.13 km/L.
UK: 45 × 0.354006 = 15.93 km/L.
No — they differ by about 20%. The identical number "45 mpg" describes a noticeably more fuel-efficient car in the US figure, purely because the UK gallon is bigger, so fewer "gallons per mile" looks like a smaller mpg number for the same actual fuel consumed relative to the US gallon definition. Always check which gallon a source means before comparing mpg figures from different countries.
km/L = 100 ÷ 6.2 = 16.129 km/L.
mpg (US) = km/L ÷ 0.425144 = 16.129 ÷ 0.425144 = 37.94 mpg.
- Averaging L/100km values directly across a trip with very different distances at each fuel economy — because L/100km is an inverse quantity, the correct average must be weighted by distance (or better, add up total litres used and total km covered, then divide), not a simple arithmetic mean of the L/100km figures.
- Assuming "higher number = better" for every unit. It's true for km/L and mpg, but false for L/100km, where lower is better.
- Comparing a UK mpg figure to a US mpg figure as if they used the same gallon — they don't, and the gap (≈20%) is large enough to change which car looks more efficient.
- Rounding the conversion factor too early (e.g. using 0.43 instead of 0.425144) and compounding the error across a multi-step calculation.
| Conversion | Formula | Where it appears |
|---|---|---|
| km/L → L/100km | 100 ÷ (km/L) | Indian and European spec sheets; unit-conversion questions |
| US mpg → km/L | mpg × 0.425144 | Comparing an imported US-market car's rated mileage |
| UK mpg → km/L | mpg × 0.354006 | Comparing a UK-market car's rated mileage |
| L/100km → mpg (US) | 235.215 ÷ (L/100km) | Quick spec-sheet comparisons |
Need the numbers instantly instead of by hand? The calculator suite's unit converter has a dedicated Fuel Economy category that handles all of these conversions directly.
Open the Fuel Economy Converter →For a structured foundation in the dimensional-analysis skills behind conversions like these, ABC Chemistry runs Class 11–12 chemistry coaching at its Gurugram centre and online across India — see abcchemistry.in for details.