Adam,
The pressure limits on the tire sidewall have no bearing on the optimum pressure for your car. The tire (manufacturer) doesn't know what car the tire is going to be installed on. The car manufacturer states the recommended tire pressure for the original sized tires for a car on a placard on the door jamb (Ford), trunk liner (GM) or owners manual. The OEM tire's specified pressure on the Gen I and Gen II SHOs was 35 psig (pounds per square inch, gauge).
I have found with the wider 225/55ZR16 tires that I use will not wear correctly a this pressure. Not only that, they will ride like rocks. At 35 psig, the center rib area will carry too mich of the car's weight and wear much more quickly than the edges of the tread. 32 psig is the upper limit for my car and my tires. 30 psig is the lower limit. I try to maintain 31 psig in my tires.
It is not the tire that supports the weight of the car. It is the air pressure in the tire. For a car that weighs about 3,500 pounds, there must be 35 psig x 100 sqin of tire touching the ground to support the car. At 30 psig, there must be 116.67 sqin of tire on the ground. That is the physics behind it. It is also the sidewall reinforcements and the tire pressure that keeps the tread flat on the pavement during high G side loads (turns).
In reality, there are significant differences in the design of tires. For example, the run-flat tires can support the weight of the car with the reinforcements in the sidewall without any air pressure. Most tires though, need a specific amount of air pressure to provide the optimum handling or ride. It is usually not the same pressure. The 35 psig that Ford specified for the SHO leans much farther towards the handling end of the range than the ride.
The answer to your question is, "it depends." It depends on the tire tread design, the tire construction, the tire size, the loading of the car on the tires, the suspension settings, the desired handling and ride compromise. I think you get the point. Play with the tire pressures to see if you can tell the difference. You should keep the pressure in the range of greater than about 28 psig to less than the limit on the tire sidewall.
Did you know that for every 10 degrees the temperature drops, the pressure in the tires drops by about 1 pound? Nice thumb rule. So when that cold front hits and drops the temperature 40 degrees, you just lost about 4 pounds of pressure in each of your tires.
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Scott
1991, 252K miles, glass hood, police grill, SVO shifter, Catz fogs,
K&N, 73mm MAF, Superchip, PP Y-pipe, Borla cat-back, 190 lph pump
Eibach/Tokico/polyurethane, SHO Shop front & rear strut braces,
16x7.5" Moda R1, 225/55ZR16 Bridgestone RE730, -1 deg camber x 4,
Class II hitch, Silver award at the SHOklahoma Car Show
