Honda did not use metric tires on the original bikes. And, I've never seen any engineering data from Honda of that era that is applicable to metric tires manufactured in the last decade or prior.
The only engineering sata I've found available for modern tire fitment is from the tire manufacturers. The engineering data is not always easy to obtain for the MFGr, as the marketed public's eyes generally glaze over when presented a list of numbers. And, such data often interferes with salesmen making higher profit margins on the larger sized tires. Salesmen prey upon the vision driven buyers who think a bigger tire on their bike MUST be better. A clever salesmen will allow or guide the buyer toward that decision.
Anyway, most MFGrs do list the rim width recommendations for the tires they offer. I've found that the strict application of inch/metric measurement numbers do NOT coincide with MFGs metric replacement recommendations. But, so far, all replacement recommendations do adhere to the rim width guidelines for their tire offering.
On the CB550 the front and rear rim width is 1.85 inches.
I *think* the rear rims of CB750 rear rims were 2.15, with the fronts 1.85, at least until 77. My 77 and 78 F comstars have 1.85 front and 2.15 rear rim widths to coincide with the 3.25H front and 4.00H tire fitment.
Some here have reported that the rear 17 inch wheels of the K models were 2.50 inches in rim width. I have none available to measure. But, the shop manual does specify a wider front and rear tire than for years prior, to the F2/3 model. So, that is not too hard to accept.
After years of trying to adapt metric tires onto my 550s and suffering through poor handling, and rapid tire wear, I gave up, and now only put stock, or very nearly so, sizes on these old Hondas with the narrow rims. For me, they handle far better, and provide predictable, even wear over an increased lifetime of use. YMMV I rue the day when Metzler stops offering inch sized tires. Hopefully, other brands will still offer the proper sizes.
Cheers,
