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Even though in theory the width of tires that have the same size molded on them should be the same, they are not. The width of tires from different makers, and even different model tires from the same maker can vary quite a bit even if the tire sizes are marked the same. Below are two front tires from Michelin. Both tires are marked 100/90-19. In theory they should be the same width, but they are not. Even when mounted on the rim the difference in tire width was obvious. I don't really understand why tire manufacturers aren't more consistent, but my experience has been that the actual tires widths can and do vary quite a bit even for tires marked the same. Ed

650ed wrote: Even though in theory the width of tires that have the same size molded on them should be the same, they are not.

Yeah, the variation is pretty clear from the specs if you look at a bunch of different tire manufacturers and models. Seems like there is a lot of marketing in those sizes.

What I was trying to say was that my guess is that the original rear tire was probably closer to a 110, even through the modern "4.00-18" tires appear to be closer to a 120. My guess is that since a 160 rear is now considered a skinny tire, tire companies assume that no one would be happy with a 110 tire for the rear of their bike, so they pushed them as much as they could for a 2.15" rim.