Just a comment from the (western)Great White North...

Under the rules for the Western Canada Motorsports Association. We allow any diameter, but rim width must follow the same maximum width as SCCA specifies for their classes (ITS&ITA=7"; ITB&ITC=6")

It certainly allows flexibility for not having to buy sometimes more expensive rims.

If you are switching tracks and are wanting to change your gearing via tire OD (ei. 225/50-14 vs 225/50-15)

Before last weekend, I realized I may not have enough tire for 4 hours of track time. (Canadian GT Championship) So, I scrambled, and found there was no 225/50-14 Toyos or Kumhos in North America through the usual sources, but I was able to get a set of Ecstas (225/45-15). Almost same OD as the 225/50-14. In a crunch, I could even run these 14 and 15 tires on the car at the same time. Yes, the handing changes, but they will get you by if the need arises. Without this flexiblility, I would have been hooped for the weekend.

By the way...The ecsta is worth at least 1.5 second over the Toyo at my home track in Calgary.

I can't think of any REAL reason to not allow the wheel diameter flexibility.

Just a comment from someone that may have froze the brain too many times....