Tire Pressure ?

[OrangeStreak]

Old thread, but I wanted to add a little more.
Larger tires have more surface area.
Per Square Inch. PSI
1 square inch at 35 PSI has less load capacity than 2 square inches at 30 PSI.
Skinny bike tires may need 90 PSI, and big wide tires may only need 25 PSI.
100 lb woman on a ¼-in Stiletto heel: Exerts 1,600 pounds per square inch (psi)

It seems like it should be the other way around when thinking in terms of volume but like you say it isn't. I run 37 PSI in my 265/70/17's and 40 PSI in the 29x2.40 bike tires which are rated to a max of 50 PSI. And as you mentioned some of the road bikes with skinnier tires take even more air. It would be treacherous to be stepped on by a woman with stilettos on...

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[LostMy65]

I understand that. Maybe the thinking is bigger wheels need more air. That is correct. But the air pressure can be less with bigger tires because they have more square inches meeting the pavement.
If 100 pounds is supported by 1 square inch, you have 100 PSI. If you have 100 pounds supported by 2 square inches, then you have 50 PSI.
It isn't perfectly linear like that with air in wheels, but it should illustrate why bigger wheels may take less air pressure than smaller wheels. Yes, more volume of air, but less air pressure.