What is the difference between US MPG and UK MPG?

They use different gallons. A US gallon is 3.785 litres while a UK (imperial) gallon is 4.546 litres, about 20% larger. Because the imperial gallon holds more fuel, the same car always shows a higher number in UK MPG than in US MPG. A car rated 30 US MPG is roughly 36 UK MPG.

How do I convert MPG to L/100km?

For US MPG, divide 235.215 by the MPG figure. For UK MPG, divide 282.481 by it. So 40 US MPG is 235.215 / 40 = 5.88 L/100km, and 40 UK MPG is 282.481 / 40 = 7.06 L/100km. The relationship is inverse: a higher MPG gives a lower L/100km.

How do I convert L/100km to km/L?

Divide 100 by the L/100km value. A car using 8 L/100km travels 100 / 8 = 12.5 km on every litre. To go the other way, divide 100 by the km/L figure.

Why is L/100km lower-is-better while MPG and km/L are higher-is-better?

L/100km measures fuel consumed over a fixed distance, so a smaller number means less fuel used. MPG and km/L measure distance covered per unit of fuel, so a bigger number means you travel further on the same fuel. They are reciprocal ways of describing the same efficiency.

Are these conversions exact?

Yes. They use the exact legal definitions: 1 US gallon = 3.785411784 L, 1 imperial gallon = 4.54609 L and 1 mile = 1.609344 km. The familiar 235.215 and 282.481 constants are derived from those, so the only rounding is in how many decimal places are displayed.

How is the trip cost worked out?

The tool converts your distance to kilometres and your fuel price to a price per litre, finds litres used from the km/L figure, then multiplies. It also reports the cost per 100 km so you can compare cars on a like-for-like basis regardless of trip length.

Fuel Economy Converter

A fuel economy converter that turns any one of the four common efficiency units — US MPG, UK (imperial) MPG, litres per 100 km and kilometres per litre — into the other three the moment you type. It is built for anyone comparing a car advertised in one country’s units against figures they already know, decoding a trip-computer readout, or sanity-checking a manufacturer’s claim before a long drive. Travellers renting abroad, importers, and anyone moving between the US, UK and metric markets all hit the same wall: the numbers look similar but mean different things, and getting them wrong by 20% is easy.

How it works

Pick the unit your number is in and the tool funnels everything through one canonical value — kilometres per litre — then fans back out to every other unit. This single-pivot approach means the four readings are always internally consistent: convert 35 US MPG to L/100km, feed that L/100km back in, and you land exactly on 35 US MPG again.

The two tricky units are the gallons. A US gallon is 3.785411784 litres; a UK (imperial) gallon is 4.54609 litres — roughly 20% bigger. That single difference is why the same physical car reads as a higher number in UK MPG than in US MPG, and it is the most common mistake people make when comparing brochures across the Atlantic. The tool keeps the two gallons strictly separate so you never mix them by accident.

Note that L/100km is an inverse measure: it counts fuel burned over a fixed distance, so lower is better. MPG and km/L count distance per unit of fuel, so higher is better. The converter labels each card accordingly and shows the underlying km/L value in the expandable working so you can audit every step.

Worked example

Suppose a US-spec car is rated at 35 MPG (US). Dividing the constant gives 235.215 / 35 = 6.72 L/100km. In km/L that is 35 x 0.42514 = 14.88 km/L, and in UK units 14.88 / 0.35401 = 42.0 MPG. So the same car is “35 in the US, 42 in the UK” — identical efficiency, two very different-looking numbers.

Now estimate a trip: 100 miles is 160.9 km, so at 14.88 km/L it burns 160.9 / 14.88 = 10.8 litres. At a fuel price of 1.45 per litre that trip costs about 15.68, or roughly 9.74 per 100 km.

Formula note: US MPG to L/100km = 235.215 / mpg. UK MPG to L/100km = 282.481 / mpg. km/L to L/100km = 100 / kmL. The constants come from 100 x litres_per_gallon / km_per_mile.

MPG (US)	MPG (UK)	L/100km	km/L
20	24.0	11.76	8.50
30	36.0	7.84	12.75
40	48.0	5.88	17.01
50	60.0	4.70	21.26

Every figure is computed in your browser — nothing is uploaded, and your inputs are auto-saved only on your own device.