I ride a giant hybrid with tires that say SRX4 with 700x40c / 40-622 / 75 psi max. I recently had a tube go bad in the back tire. Today I replaced it with a tube I bought at the bike shop which reads 700x35-40 27x1-3/8. I got it back in and the tire back on. However, when I inflate it, 75 psi seems like no pressure at all, the tire seems flat at that pressure with my floor pump.
Am I missing something? 75 seems unrideable, when I put weight on it the tire basically collapses to the rim.