US Gallon vs Imperial Gallon

The United States kept the English wine gallon of 231 cubic inches, which was already in use when the colonies separated. Britain replaced its patchwork of gallons in 1824 with a new imperial gallon defined as the volume of ten pounds of water, which worked out to about 4.546 litres.

So the two are not a metric-versus-imperial disagreement at all. They are two different English gallons, one frozen in 1707 and one redefined in 1824, both of which survived into the present.

The table hides a genuine trap. The imperial gallon is larger, but the imperial fluid ounce is smaller. An imperial pint contains 20 fluid ounces against the US pint's 16, which is what makes the imperial pint larger despite the smaller ounce.

This is why a recipe calling for "a pint of milk" is ambiguous in a way that matters: 473 mL or 568 mL, a difference of nearly 100 mL.

Miles per gallon is the most visible consequence. The same car that manages 30 mpg in the United States is rated at 36 mpg in the United Kingdom, because the UK gallon holds more fuel. Nothing about the vehicle changed.

The conversion factor between the two mpg figures is 1.20095. If you are comparing a US review against a UK one, applying that factor is the difference between a fair comparison and concluding that the British version of a car is more efficient.

This is also why L/100 km is now the standard measure across most of the world. It has no gallon in it, so there is nothing to get wrong, and lower is better rather than higher.