Fuel Economy Converter

Type in any box — the others update. Higher MPG and km/L mean better economy; lower L/100km means better economy.

The transatlantic gap is 20.09% one way and 16.73% the other

A US liquid gallon is 3.785411784 litres; an Imperial gallon is 4.54609, which is 20.09% larger. Both are called simply “a gallon” in ordinary speech, and neither country puts the qualifier on the fuel figure, so the same number means two different things depending on which side of the Atlantic wrote it down.

The misreading runs both ways and is not symmetric. An American reading a British 50 mpg hears a car 20.09% more efficient than it is; a Briton reading an American 30 mpg hears one 16.73% worse. Same gap, two different percentages, because inverting a ratio does not invert its percentage.

One gallon was defined by a shape, the other by a substance

The US gallon is 231 cubic inches, exactly — a volume specified by a length, inherited from the old English wine gallon and never revised. The Imperial gallon was defined in 1824 as the volume of ten pounds of water at a stated temperature and pressure. Ten pounds of water is 4.535924 litres, which is close to the modern 4.54609 but not equal to it: in 1985 the definition was replaced with that flat number, so a unit that used to depend on the density of water now does not depend on anything.

Both survived the switch to metric largely intact because fuel pricing is a retail habit rather than a scientific one. The litre won the pump in Britain in the 1980s and the gallon stayed on the efficiency figure, which is why a British driver buys in one unit and judges the car in another.

Three of the twelve box-to-box round trips never lose a digit, and five lose more than half

Typing into any box here sets the other three through one intermediate figure, kilometres per litre, and everything is shown to 2 decimal places. So: read a number out of one box, type it into another, and do you get your original figure back? Running all twelve ordered pairs across their realistic ranges — 70,032 conversions, of which 29,899 fail — gives a sharper answer than I expected.

The pattern is not about which units are reciprocals of which. It is about which box holds the bigger number. Both boxes show the same 2 decimals, so the box with larger values has finer resolution, and a round trip through a coarser box loses whatever fell between its steps. The three clean pairs are exactly the three that convert upward in scale — while the worst, UK mpg to L/100km, loses 80.21%.

And for six of the twelve, the failure rate is predictable to three decimal places

Six of the pairs are related by a constant factor. For those, call the target box's scale divided by the source's k. The share of values that fail to round-trip is exactly 1 − k, and zero whenever k is at least 1. Nothing here was fitted; the formula is one line and the measurement is a brute-force sweep of every two-decimal value in range.

The litres-per-100 km pairs sit in the middle of the previous table not because reciprocals are special, but because k is not a single number for them — it varies across the range, so their rate is an average of good and bad regions rather than one clean prediction. None of this is a fault and none of it is worth more decimal places. It is simply the reason to convert once from whatever figure you actually started with, rather than hopping between boxes and treating the last one as authoritative.