When/how/why max tire pressures raised from 35 PSI to 44 PSI?

dibbons said

It used to be maximum recommended passenger tire pressures were 35 PSI, now I understand they have jumped quite a bit to 44 PSI. What has changed in tire technology to justify the higher pressures? I still run my tires at 35 PSI , seems like 40 PSI or 44 PSI would be a pretty stiff ride.

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That's what I thought when I got new tires for my Nissan truck (car tires) and they told me they were 60lb recommended.
I asked the same question about the ride and they told me the rubber used was softer.
They must have been right, because I couldn't tell any difference in the new ones with 60 over the old ones as far as ride stiffness.