What is this converter?
Converts fuel economy between kilometres per litre, litres per 100 kilometres, US miles per gallon, and UK miles per gallon. Two of these measure distance per fuel and one measures fuel per distance, so some conversions are inversions rather than simple scalings.
Use it when:
- A vehicle's figures are quoted in a unit your market does not use.
- You are comparing a European L/100 km rating against a US mpg figure.
- You need to know whether a higher number is better for the unit you are reading.
Intended for: Drivers and buyers comparing vehicle efficiency across markets.
How is this calculated?
- Fuel economy comes in two opposite kinds. Kilometres per litre and miles per gallon are DISTANCE PER FUEL, where higher is better. Litres per 100 kilometres is FUEL PER DISTANCE, where lower is better.
- The input is first normalised to kilometres per litre, inverting it if the source unit is a fuel-per-distance measure.
- That value is then converted to the target unit, inverting again if the target is a fuel-per-distance measure.
- Because of the inversion, the relationship between L/100 km and mpg is not linear: the same one-unit change means very different things at high and low efficiency.
Formulas
Distance-per-fuel to fuel-per-distance
L/100 km = 100 / (km per litre)
The inversion that flips "higher is better" into "lower is better".
Gallon definitions
US gallon = 3.785412 L; UK gallon = 4.546090 L
The UK gallon is about 20% larger, so UK mpg figures are always higher for the same vehicle.
Example
Converting 8 L/100 km to US miles per gallon.
- 8 L/100 km inverts to 100 ÷ 8 = 12.5 km per litre.
- 1 US mpg ≈ 0.4251 km per litre.
- 12.5 ÷ 0.4251 ≈ 29.4.
Result: 8 L/100 km ≈ 29.4 US mpg (or about 35.3 UK mpg).
Frequently asked questions
Why are US and UK mpg different?
Because the gallons differ. A UK imperial gallon is 4.546 litres against the US gallon's 3.785 — about 20% more fuel — so the same vehicle posts a roughly 20% higher figure in UK mpg.
Why is improving from 5 to 4 L/100 km worth more than 15 to 14?
Because the measure is inverted. Fuel saved per distance is what matters, and a one-litre improvement at a low consumption level is a much larger proportional gain. Fuel-per-distance units make this easier to see, which is why Europe prefers them.
Do these figures account for driving conditions?
No. The converter only changes units. Real consumption depends on load, terrain, traffic, speed, and how the figure was measured in the first place.
Assumptions
What this converter takes as given:
- Conversion factors are the internationally standardised definitions of each unit, not regional or historical variants.
- The value you enter is treated as an exact quantity — no measurement uncertainty is carried through the conversion.
- Results are rounded only for display; the underlying arithmetic uses full precision.
- The figure supplied is a steady-state economy value; the converter changes its units and nothing else.
Limitations
What this converter cannot know or does not model:
- The converter does not know the precision of your original measurement, so it cannot report significant figures for you.
- Units that share a name but differ by region or era (for example, different definitions of a gallon or a ton) are offered only in the variants listed in the unit menu.
- Very large or very small magnitudes are subject to the limits of double-precision floating-point arithmetic.
- Test-cycle differences (for example WLTP versus EPA) are not reconciled — converting the units does not make two ratings comparable.
- Real-world consumption varies with conditions and is not modelled.