Also, 110 in the front is WAY too big. Ask me how I know.
TT: I read in a different thread in which HondaMan recommended 3.25 front and 4.00 rear (converts to just a hair over and 80 and 100) RoadRiders because the profile works well together. I wasn't exactly sure if he was referring to 550s or 750s though. Care to shed some light?
The 550s and the 750s used very nearly the same tire sizes, until the K model changed to 17 " rears in 77. I use the 3.25 front and 4.00 rear on both my 550s and 750s. They will scrape pegs before breaking traction.
Remember the profile for inch size tires was 95-100% but profile height wasn't in any public specification until metrics became popular.
3.25W/ 3.25H tire would convert to 82.55 x 82.25 in metric. 90/90 metric would covert to 3.54 x 3.16 in an inch size for a direct conversion. But, tire makers would recommend this substitution rather than make a tire that was originally intended for the 1.85 inch rim width.
4.00/4.00 inch size would convert to 101.6/101.6 in metric. A metric spec 100/90 would convert to 3.94/3.55 in inch sizes, and a 110/90 would convert to 4.33/3.9 in inches.
I'll note that some have measured their metric tires and didn't come up with the true metric measurements marked on the tire. So apparently, the metric numbers are advisory rather than actual, at least from some manufacturers.
None of the metrics are direct substitutions for the inch sizes originally specified. The tire salesmen don't care if you pound a square peg into a round hole as long as you pay their commission.
After years of accepting tire "upgrades" offered by tire salesmen, I finally went with manufacturers that make tires in inch sizes for these 70 bike's. They work well. And work better over their operating life than the metric ones that are "close enough", imo. All new metric tires fitted work better than what was replaced for the short term. Then you gradually get used to strange wear patterns while riding and adapt your riding habits. Next time a tire replacement happens, you are so impressed with the new ones, that you think they must be *that* much better than those replaced, until wear happens.
Cheers,