Weird historic measurements in the UK: Miles per Gallon

OK so in the UK why do we use the performance term ‘miles per gallon’? I’m guessing the reason is simply that we haven’t updated it…

MPG is the number of miles covered per gallon of fuel, usually used as a performance measure in cars or other road-based vehicles. However we certainly do not buy petrol in gallons any more. The last time we used gallons as a unit of measurement at petrol pumps was over 30 years ago.

Therefore why don’t we just use miles per litre or MPL? Just a suggestion…