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...the wheel rim and tire on a bicycle add up to a combination system. Sometimes, with a 50 year old wheel rim, you cannot take advantage of a more modern, high pressure bike tire's construction, that allows higher pressures. Other than that, Panaracer generallly makes better tires for bikes than Kenda. But both companies sell a wide range of products...just not in the 27" department. It's entirely possible that your 27" rims on a Schwinn take a proprietary tire. I don't know what came standard on the SS 50 yers ago.
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