The auto manufacturer, but not without the tire manufacturer's blessing. The number on your tire represents "maximum" pressure that it can withstand for safety. But you have to keep in mind that a certain tire can be mounted on several different vehicles, and the tire company has no way of knowing which one you're putting their product on. The automotive designers work with tire manufacturers during the design and testing phase of a vehicle. BOTH come to a conclusion as to what should be the recommeded tire pressure for YOUR vehicle for maximum safety and intended performance. Trust the door placard.

Actually, you should follow the recommendation of the tire manufacturer for two reasons. First, aftermarket or replacement tires will probably have different pressure ratings than the original equipment tires. The manufacturer of the tire has completed extensive testing to ensure the optimal pressure for that particular tire design. Different rubber compounds and other aspects of tire production will lead to different air pressure requirements. Second, the vehicle manufacturer has completed extensive testing of their own geared towards safety AND ride quality. Often, this results in a compromise; lowering tire pressure to improve ride quality at a slight sacrifice of safety. The Ford Explorer debacle is an example of this problem, although it was compounded by other issues. The OE tires were designed to have a higher psi rating than was used. Coupled with high vehicle weight, center of gravity, and just plain inattention to tire pressure on the behalf of vehicle owners, a recipe for disaster was created. Generally, however, I would say that the difference between the OE recommended tire pressure and the replacement recommended tire pressure will create little problem. In a car, heavy loads are not usually an issue, so the risk is very small, a small drop in PSI from the recommended will simply provide a better ride quality. In a pick-up truck, conversion van, or large SUV, I would stick to the tire manufacturer's suggestion. The vehicles themselves are heavier and often carry more of a load. Weight plays a large role in the effect of lowered tire pressures on safety. Speed is also a factor to consider, the faster you go, the hotter the tires get (and more likely to fail!), especially when they are low. The most important issue, however, is to check your tire pressure regularly. Pressures vary widely with temperature changes. Barely noticeable, leaks from a valve stem or screw in the tire, can go unseen for days or weeks while seriously impairing the safety of the tires, especially during any panic maneuvers. The best way to ensure the safety of the tires, once you have determined which pressure to stick with, is to make sure that they are always inflated to that PSI.