I've always thought they're meant to be around 32-36 psi. Often 35 psi being an exact figure. I too am wondering this for my new 225/65/r17 tyres cos I checked the pressures other day and they were all different. 35, 37, 38 and 40. Dno what the tyre place pressurized em to.

 

Yeah the tireplace that installed them set em to 38 PSI , I deflated to 32 PSI, drove around a bit and it felt squishy, so I reset them to 36 PSI seeing as I do a lot of road/highway miles and that seems to be the sweet spot, there’s a little give but not hard as a rock when going over bumps