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.
The tube size shouldn't affect the feeling of the tire at all (Unless the walls stretch too much and blow out... But that's a different matter). It seems like something is preventing the air from flowing inside the tube itself and it is being trapped somewhere between the tube valve and the pump. You can test this just by plugging the valve in the pump, does it fills the barometer? If not, the reading you are getting when pumping up is from air trapped (And pressurized)somewhere else than the tube.
Take it all off again and check the tyre thoroughly, when this happened to me I thought I'd bought a dodgy Chinese tube, so I got another. The second time I ran my fingers over every inch of the inside while flexing the tyre open and found a tiny bit of wire just barely poking through the tyre into the tube and dug it out.
Is it a presta valve? If so, you might have forgotten to unscrew the small nut at the end. This would prevent any air from getting in the tube. It is also possible that you did not push the pump all the way onto the valve (this could happen with any type of valve).
In either scenario, you would be pressurizing the pump to 75 PSI, but not putting any air into the tube itself.