"Am I missing something in the calcution of converting inches to center with 2.54 x 1 inch?"
What I think you're missing is the change in sizing that occured during the period around the ~ late 1980's.
The earlier sizing, which many of the later 27" tires (still?) adhered to had 27X1" tires measuring only 20mm wide. Many later tires had both the 1" (printed) nomenclature along with the defining "20-630" in tiny raised letters ON THE SAME TIRE.
700c tires converted to more accurate sizing within a much narrower time frame. Earlier 25mm tires measured 20mm wide and some even had "700X25" and "20-622" (again in raised letters) on the same tire.
Today's 700c tires are very accurate by comparison, with only a maximum of 1mm undersize for a "23mm" tire. An exception is Continental, and if I recall some fairly recent Panaracer and Avocet tires: Their 28mm tires seem to run about 25mm wide when new, and their 25mm tires about 24mm wide. Michelin, OTOH, runs maybe a tad large but moreso in some larger sizes, making them an exception as well.
I find the $20 digital calipers useful for so many applications and this is yet one more. Keep in mind road tires can grow a mm or so in width over time. Also, rim width affects both height, and to a greater degree width, of mounted tires. BOTH dimensions increase on wider rims, but if the rim is relatively wide already then height hardly changes at all when rim width is increased further.
David Snyder Auburn, CA usa
Lou Deeter wrote:
> ...1. If I compare 27x 1" tires to 700x25, one would think that the
> 700x25
> would look thinner, but not my experience. Likewise, 27 x 1 1/8 seems
> narrower
> than 700x28 and 27 x 1 1/4 certainly seems narrower than the Gran Bois
> 700x30.
> Am I missing something in the calcution of converting inches to center
> with
> 2.54 x 1 inch?