You will hear people say to go by the placard and disregard everything else. With our old GTOs, it might not be the best idea. Even on radial tire equipped cars the pressure on the glove box door is a compromise between ride quality, handling, tread wear, and economy, and they are put in roughly that order, but always ride quality is first. They want it nice and smooth so you dont feel tar strips, and you sacrifice the other ones to get that soft cushy ride.
On the sidewall of your tire you have some information. Load capacity and max pressure, or recommended pressure on some tires. The load capacity goes up with more air pressure, and the ride gets stiffer, but you have less rolling resistance and you get better mileage to a certain extent.
I tend to run just under the max pressure, and even with 40-45 series tires (the short ones) I get even wear across the tread and long life out of them. The Eagle F1 tires I had on my 98 Formula went 70,000 miles, and the main reason I changed them was they were weather checked from sitting outside since 2005.
I go by the sidewall and I have yet to go with anything under 40psi. The ride is a bit stiffer, but the tires last, I get better mileage, and it handles great.