Is a UK gallon bigger than a US gallon?
Yes, by about a fifth. The imperial gallon used in the United Kingdom is exactly 4.54609 litres. The US gallon is exactly 3.785411784 litres. Divide one by the other and the imperial gallon is 1.200950 times the size, or 20.095% larger. A car covers 20.095% more miles on an imperial gallon than on a US one.
Both numbers are definitions rather than measurements. The US gallon is 231 cubic inches, and since the inch was fixed at 25.4 millimetres in 1959 that volume has been 3,785.411784 cubic centimetres with no rounding. The imperial gallon was set directly in litres by the Weights and Measures Act 1985. Neither will change, so every conversion on this site is exact to whatever precision you ask for.
Why are there two gallons at all?
Because Britain redefined its gallon in 1824 and the United States, independent for fifty years by then, kept the old one. The American gallon is the English wine gallon of 231 cubic inches, standardised under Queen Anne in 1707. The imperial gallon was a fresh definition: the volume of ten pounds of water at 62 degrees Fahrenheit.
Before 1824, England used several gallons at once. The wine gallon of 231 cubic inches, the ale gallon of 282 cubic inches and the corn gallon of about 268.8 cubic inches all coexisted, each tied to a trade. The 1824 Act swept them away in Britain and its colonies and replaced them with one imperial gallon, defined by weight of water rather than by a box of a given size.
The United States never adopted the 1824 Act. It had inherited the wine gallon for liquids and the Winchester bushel for dry goods, and it kept both. That is also why the US has a separate dry gallon of 4.405 litres, a unit almost nobody meets outside agriculture. Britain has one gallon; America has two, and the liquid one is the one on every pump.
Canada, Australia, New Zealand, South Africa and Ireland all used the imperial gallon as British territories and all abandoned it for the litre between the late 1960s and the 1980s. Only the UK and a few small Commonwealth and Caribbean jurisdictions still quote fuel economy per imperial gallon, and even the UK sells the fuel itself by the litre.
How much does the gallon change an mpg figure?
By exactly 20.095%, always. Multiply a US mpg figure by 1.200950 to get the imperial figure, or divide an imperial figure by 1.200950 to get the US one. 30 mpg (US) is 36.03 mpg (UK). 50 mpg (UK) is 41.63 mpg (US). The car has not changed; only the size of the gallon in the denominator has.
Because miles cancel out of the calculation, this is the one fuel economy conversion where the mile plays no part. It is a pure ratio of the two gallons. That makes it the simplest conversion on the site and also the one most often skipped, because both figures carry the same label, mpg, and nothing on the page warns you that they are different units.
The practical effect is that British fuel economy figures always read higher than American ones for the same car. A British reader browsing US reviews will think every American car is thirsty; an American reader browsing British reviews will think every British car is a hybrid. Neither is true. The MPG US to UK converter resolves it in one step.
Why do imported spec sheets look better than the car?
Because a figure quoted in imperial gallons is 20% higher than the same efficiency quoted in US gallons, and a listing rarely says which gallon it means. A Japanese-import people carrier advertised in Britain at 30 mpg might be 30 imperial, which is 24.98 US and poor, or 30 US copied from an American forum, which is 36.03 imperial and respectable.
The ambiguity cuts both ways and the money involved is not small. At 12,000 miles a year and 1.50 a litre, the difference between a real 30 imperial mpg and a real 36 imperial mpg is roughly 300 a year in fuel. Anyone buying a grey import, a classic with figures from an old American road test, or a car reviewed abroad should convert the figure explicitly and write down which gallon the source used.
The same trap catches the official numbers. An EPA combined rating of 32 mpg and a UK WLTP figure of 38 mpg for the same model look far apart until you convert: 32 US is 38.4 imperial. Once the gallon is accounted for, the remaining gap is the difference between the two test cycles, which the ratings guide covers.
Which gallon does each source use?
American sources use the US gallon, without exception. The EPA, fueleconomy.gov, every US manufacturer's website, every US magazine and every US window sticker mean 3.785411784 litres when they say gallon. British sources use the imperial gallon, also without exception: the Vehicle Certification Agency, UK manufacturer sites, Autocar, What Car, Parkers and the government's fuel economy label all mean 4.54609 litres.
The grey area is everywhere else. Canadian documents from before the 1970s mean imperial. Australian and New Zealand documents from the same period mean imperial. Anything from Ireland or South Africa before metrication means imperial. Modern documents from all of those countries use L/100km and avoid the question entirely, which is the sensible answer and the reason the L/100km converter offers both gallons.
Online forums are the worst case. Owners post figures from their trip computers without saying where the car was bought, and other owners reply from other countries. If a forum figure matters to a decision you are making, trace it to its source before you convert it.
How do I convert between the two gallons?
For mpg: multiply US by 1.200950 to get imperial, or divide imperial by 1.200950 to get US. For volume: a US gallon is 3.785411784 litres and an imperial gallon is 4.54609 litres, so multiply US gallons by 1.200950 to get imperial gallons. The gallons to litres and litres to gallons pages handle volume with a toggle for each.
For L/100km, the two constants are 235.214583 for US gallons and 282.480936 for imperial ones. Both are 100 times the gallon in litres divided by 1.609344, the kilometres in a mile. Divide the constant by the mpg to get L/100km, or divide it by the L/100km to get mpg. The UK mpg to L/100km page uses the imperial constant throughout.
A rule of thumb for the head: a US mpg figure is about five sixths of the imperial one. 48 imperial is 40 US; 60 imperial is 50 US; 36 imperial is 30 US. It is close enough for reading a review and wrong enough that you should use the converter for anything you will act on.
The habit that stops the mistake
Label every mpg figure with its gallon, every time you write one down. 40 mpg (US). 48 mpg (UK). The parenthesis costs four characters and removes the ambiguity for everyone downstream, including you in six months. This site does it in every table, every converter and every sentence for that reason.
If a figure arrives unlabelled, find out where it came from before you trust it. American source, US gallon. British source, imperial gallon. Anything else, look for a L/100km or km/L figure alongside it, which will settle the question, since those units are the same everywhere. Litres are litres; only the gallon has two sizes.