You know where it gets all crazy-pants? Tires. When I bought tires for my first car, they were … um… L-15s, I think. But by the second time I bought tires, they’d switched to the current system. In the current system, the tire’s inside diameter (rim diameter) is measured in inches, its widest point (which falls at an unspecified place) is measured in millimeters, the sidewall height has to be calculated from the “aspect ratio” or “series profile” which is specified as a percentage (so, a P205/70R15 has a sidewall height that is 70% of 205mm I kid you not) and the top load and speed ratings are specified by completely arbitrary number and letter codes, respectively. The positions of the ratings are fixed unless it’s a Z rated tire in which case the speed rating is moved over to the middle between the aspect ratio and the rim diameter, next to the construction rating (which is usually R for Radial these days).
I swear to God I did not make any of that up.
