Just curious. I am replacing some tires on my plow truck. It’s old. A 1976 Chevy. I put 10.5 inch wide by 31 inch tall on 15 inch wheels.
The truck tires are basically a LT 275 75R 15. The width is in metric and the wheel size is in inches. The 75R is the sidewall height relative to the width of the tire.
What’s weird is that new tires are specked out with a combo of metric and inches.
How are tires sized in countries that use the metric system? Are wheel sizes in metric or inches? Or both.
I also put new tires on my Pathfinder. LT 265 75R 16. Again with the mix of metric and inches. 265mm wide. 16 inch wheel.
Going from memory here.
For reasons I am not quite sure why rims sizes settled on inches. I do seem to recall seeing some listing for old French cars (1960s and earlier) that called for either 380mm or 400mm wheels.
In the 1960s metric sizes started to appear in tire stores for imported cars. A 165X15 was a very popular size used on Volkswagens and many other imported cars. Back then American cars used strictly inch measurements for tires like 6.50-13. The 6.50 was the measurement in inches of the width of the tire. The aspect ratio was about 83%.
Then came low profile tires with alpha sizing. G78-15. G was bigger than F and smaller than H. 78 was the aspect ratio. A 70 series was wider and lower than a 78 series, and that was wider and lower than the previous 83% tire.
Then came P-metric/metric sizing. The letters were dropped, and the size (except for rim size) was expressed in metric. P225 75R16 for example. The 225 is the size, the 75 is the aspect ratio, the R is for radial, and the 16 is rim size. P means passenger car tire. In the OP example LT is light truck. Speed rating will also be listed with the R such as SR, HR, ZR
When you get into truck tires, the sizing goes a little different. 31 10.50 15 would be a 31" tall tire, 10.5" wide with a 15" rim.
Anyway getting back to the OP question, why inches for rims? Honestly I have no idea except for after WWII and up to the 1960s America and England did a majority of all car making and they both used inch sizing. Other car makers from other countries found it easier to use existing size wheels and tires than it was to get a tire maker to come up with a new (hard to find) tire for a new model. It was the rise of the beetle that brought out metric sizing, but I think rim size by that point was pretty well entrenched.