Gallon is a measure of what?

In the U.S., we still use gallons for gasoline, bottled water, and AFAIK, milk/juice. oddly, large bottles of soda, like Coke switched to 2 & 3 liter sizes years ago. If the government tried to switch gasoline to liters, there'd probably be a revolt. Speedometers have shown both mph and kph for decades, though I don't know why.

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