I remember back in the 70's when I was in grade school, there was a big push to convert over to the metric system and we had to learn a lot of it. I seem to remember there being a certain date that the US was supposed to actually switch over...but it never happened.
So what did we end up with..... Tool sets with SAE and metric, and tire sizes.
Like say your tire size is 225/75R15. That means it's section width is 225 millimeters (metric), but it's rim diameter is 15 inches. This is why I can spurt off that there are 25.4 mm's to an inch in my sleep.