Fuel economy converter

What is a fuel economy converter?

A fuel economy converter is a free online tool that translates a vehicle’s efficiency from one unit into all the others. Enter a value and choose the unit it is expressed in — miles per US gallon, miles per UK gallon, litres per 100 kilometres, or kilometres per litre — and the converter shows the equivalent figure in every other unit at once. It is useful when comparing cars advertised in different regions, reading a spec sheet from abroad, or planning a trip where the local unit differs from the one you are used to.

The tricky part of fuel economy is that the units do not all point the same way. With mpg and km/L, a bigger number means a more efficient car. With L/100 km, a smaller number is better, because it measures how much fuel is burned to cover a fixed distance. The converter handles that inversion for you.

How does it work?

Every value is first converted into a common base of litres per 100 km, and then out again into the three remaining units. Because L/100 km is inversely proportional to the distance-per-fuel units, the conversions to mpg and km/L use division rather than multiplication. The exact constants come from the size of each gallon: a US gallon is about 3.785 litres and a UK (imperial) gallon is about 4.546 litres, which is why the two mpg figures differ noticeably for the same car.

Formula

To convert an input into the base unit of L/100 km:

$\text{L/100 km} = \frac{235.215}{\text{mpg (US)}} = \frac{282.481}{\text{mpg (UK)}} = \frac{100}{\text{km/L}}$

A value already in L/100 km is used directly. From the base value, the other units are:

$\text{mpg (US)} = \frac{235.215}{\text{L/100 km}} \qquad \text{mpg (UK)} = \frac{282.481}{\text{L/100 km}}$

$\text{km/L} = \frac{100}{\text{L/100 km}}$

Worked example

Take a car rated at 30 mpg (US). First convert to the base unit:

$\text{L/100 km} = \frac{235.215}{30} \approx 7.8405$

Then convert out to the other units:

$\text{km/L} = \frac{100}{7.8405} \approx 12.75 \qquad \text{mpg (UK)} = \frac{282.481}{7.8405} \approx 36.03$

So 30 mpg (US) is about 7.84 L/100 km, 12.75 km/L, and 36.03 mpg (UK).

As a second example, a car listed at 8 L/100 km converts to about $\frac{235.215}{8} \approx 29.40$ mpg (US) and $\frac{100}{8} = 12.5$ km/L.

Conversion reference

Notes

Notice how the same car looks thriftier in mpg (UK) than in mpg (US) — not because it uses less fuel, but because the imperial gallon is larger. When you compare vehicles, make sure the numbers are all in the same unit first. For working out a car’s economy from a trip rather than converting a rating, use an MPG calculator, and to estimate the fuel and cost of a specific journey, try the fuel consumption calculator.

FAQ

Why are mpg (US) and mpg (UK) different?

Because the two gallons are different sizes. A US gallon is about 3.785 litres, while a UK (imperial) gallon is about 4.546 litres. Since the UK gallon is larger, the same car travels farther on it, so its mpg (UK) figure is roughly 1.2 times the mpg (US) figure.

Is a higher or lower number better?

For mpg (US), mpg (UK), and km/L, higher is better — the car covers more distance per unit of fuel. For L/100 km, lower is better, because it measures the fuel burned over a fixed distance.

How do I convert L/100 km to mpg?

Divide 235.215 by the L/100 km value to get mpg (US), or divide 282.481 by it to get mpg (UK). For example, 7.84 L/100 km is about 235.215 ÷ 7.84 ≈ 30 mpg (US).