Why is my mpg different from a British magazine's figure for the same car?

Because the gallons are different. A US gallon is 3.785 litres, an imperial gallon 4.546 — about 20 % larger. The same car that returns 28.1 mpg in the United States returns 33.7 mpg in the United Kingdom without anything mechanical changing.

How do I measure a tank properly?

Fill the tank, reset the trip meter, drive normally, and fill it again at the same pump setting. The distance on the trip meter and the litres or gallons of the second fill are exactly the two numbers this tool wants. One tank is noisy; three tanks averaged is a real figure.

Is lower better, or higher?

It depends on which way the unit runs. Miles per gallon and kilometres per litre measure distance per fuel, so higher is better. Litres per 100 km measures fuel per distance, so lower is better — and that inversion is why the two scales cannot be compared by eye.

Why does the tool show a cost per 1 000 km?

Because consumption on its own is abstract. Multiplied by the price you paid, it turns into the figure that decides whether a longer route is worth it, and it is the number to compare between two cars you are choosing between.

Gas Mileage Calculator — MPG, L/100 km and KM/L

Gas mileage calculator

320 miles on 11.4 US gallons is 28.1 mpg. The same car reads 33.7 mpg in imperial gallons and 8.4 litres per 100 km — three numbers, one tank, and the reason a British review and an American one never seem to agree.

Result28.1 mpg

28.1 mpg (US) — 33.7 mpg imperial, 8.4 l/100 km. At the price you entered that is $95.19 of fuel every 1 000 km.

Trip Card

This trip only: the numbers it produced, the limits and tariffs it was checked against, and when those were checked. Print it or send it to whoever is travelling with you.

Litres per 100 km	8.4 l/100 km
Miles per US gallon	28.1 mpg (US)
Miles per imperial gallon	33.7 mpg (imp)
Kilometres per litre	11.9 km/l
Fuel cost per 1 000 km	$95.19

Trip Card generated on September 20, 2026

One tank, four scales

Enter the distance driven and the fuel it took, and the result comes back in every scale at once: US mpg, imperial mpg, litres per 100 km and kilometres per litre. They are the same measurement expressed four ways, and a car sold in several markets is quoted in all four.

If you add the price you paid, the tool also gives what that consumption costs over 1 000 km at that price — which is the form the number takes when you are comparing two cars or deciding whether the toll motorway is cheaper than the free road.

Where the conversions come from

The definitions are fixed, not approximate: 3.785411784 litres to the US gallon, 4.54609 to the imperial gallon, 1.609344 kilometres to the mile. Everything here is derived from those three constants, so switching units never introduces a rounding drift of its own.

What the number is worth

Fuel economy measured over a single tank carries the weather, the traffic and the driver in it. Measured over three, it is a property of the car. If you are using this to settle an argument about whether a car is thirstier than the brochure claims, average several tanks before you believe the gap.

What this means in United States

US mpg uses the US gallon, which is about 17 % smaller than the imperial gallon a British or Irish reader means by the same three letters. The same car reads 34 mpg in Ohio and 41 mpg in Ireland, so the tool prints both alongside l/100 km.

Questions people actually ask

Why is my mpg different from a British magazine's figure for the same car?: Because the gallons are different. A US gallon is 3.785 litres, an imperial gallon 4.546 — about 20 % larger. The same car that returns 28.1 mpg in the United States returns 33.7 mpg in the United Kingdom without anything mechanical changing.
How do I measure a tank properly?: Fill the tank, reset the trip meter, drive normally, and fill it again at the same pump setting. The distance on the trip meter and the litres or gallons of the second fill are exactly the two numbers this tool wants. One tank is noisy; three tanks averaged is a real figure.
Is lower better, or higher?: It depends on which way the unit runs. Miles per gallon and kilometres per litre measure distance per fuel, so higher is better. Litres per 100 km measures fuel per distance, so lower is better — and that inversion is why the two scales cannot be compared by eye.
Why does the tool show a cost per 1 000 km?: Because consumption on its own is abstract. Multiplied by the price you paid, it turns into the figure that decides whether a longer route is worth it, and it is the number to compare between two cars you are choosing between.

Where these numbers come from

Last checked: September 19, 2026

US gallon 3.785411784 l and imperial gallon 4.54609 l (NIST/international definitions) — September 19, 2026

In the United States baggage allowances are the carrier's own contract of carriage; the Department of Transportation requires the fees to be published, not harmonised, so two airlines on the same route can measure the same suitcase differently.

These figures are guidance, not a quotation. Airline allowances, gate fees and toll tariffs change; check the carrier or the road operator before you travel.