If you want to see something fun, go check the size of the tires on your car (printed on the wall of the tire). It will read something like "245/35R18", where 245 is the section width in millimeters and 18 is the diameter in inches. All standard tire sizes use a combination of inches and millimeters, haha.
it gets worse the more you dig in. The middle number is am aspect ratio for length of sidewall. so its side wall thickness 35% of 245mm. Then on top of that when youre getting into trucks and jeeps common sizing switches to inches. so you go from 315/70/R18 to something like 35x12.5r18
