Okay, just about anywhere you ask this question, the answer is pretty clear:

"Do I go by the pressure number written on the car's door frame, or the pressure number written on the side of the tire?" The answer is always: "Use the number on the door frame. The number on the side of the tire is a maximum do-not-exceed number."

Okay, but here's my follow-up question that I can't seem to find the answer to anywhere:

How can this "door frame" number be true for every make and model of tire I put on the vehicle? You mean to tell me, that among all of the varied brands and types of passenger car tires available, that they all need to be inflated to the exact same pressure as the tires that came on the car when it was brand new?

You'd think there'd be some variation among brands and models, wouldn't you?

The reason I ask is because I just put new tires on my car, and as usual, the tire shop inflated them to 35psi, when my door frame clearly says 30. And even at 35psi, these new tires still appear to have a pretty significant pot-belly bulge next to the contact patch. So is my door frame still right?

Now, before I get a lecture on the pros and cons of different tire pressures, I already know most of them:

- Lower tire pressure will give a smoother ride and greater traction (contact patch is bigger), but will cause the tires to heat more quickly and wear out more quickly.

- Lower tire pressure will cause the tires to wear more quickly on the outside edges than in the center.

- Higher tire pressure will cause the tires to wear more quickly in the center.

- Higher tire pressure will make the car's handling appear to be more "darty" and "oversensitive" because the contact patch is smaller.

Really, I just want to know how that door-frame number can really be right for every single tire I could possibly put on the car.