Pete,
Please consider the possibility that a "gallon" in the U.K. may not be the same as a "gallon" in the U.S. A few decades ago we ( UK and US) tried to convert to the metric system so as to be standardized with the rest of the world, the conversion was not successful because people refuse to change. The only way I know to compare our systems at this time is by litre, unfortunately, the USA does not sell gasoline by the litre because we refuse to go along with the rest of the world's standard. Until we all can agree on a world "standard" we will continue to be confused... I think that it is all related to the length of an old King's shoe, or the weight of someone's "stone".
Not my idea of clear communication!