Gallon is a measure of what?

An Imperial gallon by definition is not measured in litres. It was and is 8 pints . It is interesting that while the change to metric dispensing of fuel was ( I think) back in the 1970s , consumption is still measured as miles per gallon. I couldn’t even begin to convert that to km per litre . Nor can I convert price increases per litre into “real” costs . Just that it is likely that even the smallest increase will be more per gallon than I actually paid for a gallon when I started driving in 1971