The US gallon was different to the gallon used in UK and Australia. The US gallon was smaller. Australia's changeover to metric was in the 1970's.

I did not know that about the UK not being fully metric other than they still had road sign posts with miles instead of kilometres. Do cars over there use KM or miles for distance and speed? And what about speed signs? Having both must be confusing for the younger generation.

So in car racing, I guess the european cars are faster because they use KM/hr instead of MPH. lol
(Oh a few of the German freeways are speed limitless I heard.)