When you size up a car tire, you use something like 195/65/15. 195 millimeters, 65% (of 195), and 15 inches. Why is it so mixed up like that? Why can’t the diameter be measured in cemtimeters, or the tread be measured in inches?
The 195/65/15 could easily be 7.7/65/15, or even 195/65/38.
My WAG would be that wheels are made in whole increments of 1" with the odd 1/2" size squeezed in there.
Tire widths generally are in 5mm or 10mm increments.
What confuses matter more is that some larger tires are all imperial. FOr example, a popular size is 31x10.5R15 (31" overall diameter, 10.5" width and 15" rim dia.).
My God! Why haven’t I ever asked this question? I’m kind of a tire fiend.
Bravo!
(My best guess: They didn’t always. Tires/wheels used to be pretty skinny and there weren’t that many sizes out there. As “modern” times rolled- no pun intended- around, the need for a better system emerged. Someone smarter, and who drinks less, will come along and clear this all up, I’m sure.)