Last Friday I put a set of Goodyear Comfort Treads on my 98 OBLW. The tire dealer filled them to 29 psi as spec'd on the door sticker, but the tires looked really flat at that pressure. The maximum recommended pressure on the sidewall is 44 psi but the dealer said that would completely wrong for the car. That got me to thinking.
I reckon that the tire is designed to hold a certain shape under load, and depending on the weight of the car that would involve different pressures for different cars, and the maximum on the sidewall would refer to the pressure at the highest rated load. Different tires also have different sidewall stiffnesses, so it doesn't make sense to go by the OEM door sticker when changing to a different brand/model tire. If I put too much pressure in the tire it will have the wrong shape and not handle correctly. But I like to have the highest reasonable pressure to decrease rolling resistance, since most of my miles are highway.
Is there a way to calculate the correct pressure for a specific tire and car weight (assuming typical passenger/cargo load and driving conditions)?