Well, as an American, I happily retain use of Imperial measure... which I suppose, ought to be renamed American measure, as we are SO reluctant to go metric here, lol. but to answer jno's statement, there IS a decimeter between a centimeter, and a meter.. and our metric rulers ARE marked with them. I would say the tendency to retain "mpg" is an American influence, and I would imagine the weatherfolk are giving both, as an aid to transition.
I imagine our sayings will just shift, slightly, or drop out of usage. Or, remain the same, we still say, "I love you a bushel and a peck", and I have no concept of how much either actually holds, lol!
miles per gallon works out to a higher figure than kilometers per hour (I think?) so, that makes it sound better, just like $2.99 sounds better than three dollars.
However, all the people I know who use kilometers, call them "clicks", whether it be clicks per liter, clicks per hour, or just so many clicks distant. ...continued....