Fuel cost and economy calculator
What a journey costs in fuel — with your economy shown in all four units, because the two gallons are 20% apart and miles per gallon is a reciprocal that does not behave the way anybody expects. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
There are two gallons, and they are 20% apart
The imperial gallon is 4.54609 litres. The US gallon is 3.785411784 litres. Both figures are exact — they are definitions, not measurements — and they differ by 20.09%.
So “40 mpg” is two different cars:
- 40 miles per imperial gallon is 33.3 miles per US gallon
- 30 miles per US gallon is 36.0 miles per imperial gallon
A review, a spec sheet or a forum post quoting mpg without saying which gallon is wrong by a fifth for half its readers. This page always says which, and shows both.
Five more mpg is not five more mpg
Miles per gallon is miles divided by fuel. That makes it a reciprocal of the thing you buy, and reciprocals do not add up the way intuition expects.
Over 10,000 miles, improving by exactly 5 mpg saves:
- 10 → 15 mpg: 333 gallons
- 20 → 25 mpg: 100 gallons
- 30 → 35 mpg: 48 gallons
- 40 → 45 mpg: 28 gallons
- 50 → 55 mpg: 18 gallons
Eighteen times as much fuel saved at the bottom as at the top, for the identical change on the dial.
The practical consequence is the one nobody expects: replacing a thirsty vehicle with a slightly less thirsty one saves far more than replacing an efficient one with a very efficient one. A 10 mpg van going to 15 saves more than three times what a 40 mpg car going to 50 saves — and the second improvement looks twice as impressive.
Which is why the rest of the world uses the other unit
Litres per 100 km is fuel divided by distance — the right way up. It is linear in the fuel you actually buy: a drop of one litre per 100 km saves the same amount of fuel wherever you start from, so the number behaves the way people already think it does. Miles per gallon does not, and that is the whole difference.
Both are on this page for every figure you type, because the conversion is the point: 45 mpg and 6.28 L/100 km are the same car, and only one of them can be reasoned about by subtracting.
No magic 282.481
The factor for turning mpg into litres per 100 km is usually quoted as 282.481. It is not a constant of nature; it is
100 × 4.54609 ÷ 1.609344 = 282.480936…
This page derives it from the mile and the gallon every time rather than storing the rounded version, because a hand-typed constant is how a page ends up with a digit nobody can trace. The US figure, 235.214583, comes out of the same expression with the other gallon.
What this cannot tell you
It does not know your actual consumption. A manufacturer's figure comes from a standardised test cycle, and real driving — cold starts, hills, a roof box, traffic — routinely differs from it by a margin far larger than the differences people shop on.
If you want a figure that means something, fill the tank, drive, fill it again, and divide. Type that number in here. The arithmetic on this page is exact; what it is exact about is whatever you gave it.
The mile (1609.344 m), the imperial gallon (4.54609 L) and the US liquid gallon (3.785411784 L) are all exact by definition. Every other number here is computed from those three.