My car has both measurements, and it is an *ancient* 1982 model Holden. I'm in Oz - my generation grew up with the old Imperial measurements, but we went metric long ago. All I can say really, is that after a while you develop strategies to deal with it, and you get used to it. I wouldn't have it any other way now.
It is useful to be familiar with how they relate to each other approximately when you come across things like old cookbooks, and I must admit that when it comes to lengths, such as working out how my furniture is going to fit into a room, or looking at a folded tarp which is 900cm x 600, I quickly convert to a rough approximate measurement in feet, by dividing by 30 (approximately the numbers of cm in a foot) I can visualize the size of something that is 30 x 20ft much better than the other way - or I can look at a garden stake that is 2100mm and think - yeah, that's a 7 ft garden stake. So I guess I have my feet in both camps, but I'm quite comfortable that way.