OK, just to sanity check, because it's not clear from the comments below.

We all realize that metric areas do use hp for car engines as well, right?

And a lot of them also do inches for TVs, which is weird and forces you to go digging into the specs for the cm measurements whenever you want to see if a TV will fit in a space.

EDIT: Oh, I'm wondering now, do people use liters/cc for engine volumes in the US? I don't know, but I also haven't ever heard of a different way to refer to engine volume ever, so they must. What would they use instead?

EDIT 2: For my money the most annoying unit conversion in car measurements is the US going for miles per gallon, keeping the volume of fuel constant and giving you the distance while metric uses liters per 100km, keeping the distance and giving you the volume of fuel. It may as well be impossible to convert between the two.