The "correct" number is the one on the car.
What happens if you change tire sizes from what the car shipped with? Is there some way to calculate a good "recommended" PSI? I went from 235/50-18 to 255/45-18 on my car, and stuck to the numbers on the door plate.
In my racing experience, we'd start high and lower the pressure until the cornering wear on the side of the tire started to get around 1/4 inch. This way we kept the tire from 'folding' (underinflation) and also maximized the contact patch.
It's really a trial and error thing.
You have to base it on your driving style, the conditions you're in, and the tire itself.
I encourage everyone to pay more attention to their tire pressures. A simple change of 5psi could be as important as a $500 tune-up wrt fuel mileage.
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