Dear Tom and Ray:

I've got a 1990 Buick with a new set of tires. The sidewall says 45 psi, and the mechanic put in 40 psi because of the rating on the sidewall. On the driver's- side door, it says to put 30 psi in the tires. The mechanic says that only applies to the original tires. Who's right? The door or the mechanic? -- Doug

RAY: Well, isn't this a tough one? Which is smarter, a mechanic or a doorjamb? Can I get back to you on that?

TOM: Well, let's start by giving the mechanic partial credit. He's right that the rating on the doorjamb is for the original tires. And if you replace your original tires with a different type of tire, the optimal pressure might be different, too.

RAY: But overall, I think we're going to have to give the nod to the door. If you checked with Buick and asked what size replacement tires are acceptable for this car, Buick would probably recommend the original size, or something very close to it. Whatever Buick recommended would NOT be so different as to increase the optimal tire pressure from 30 to 40 -- which is a huge increase.

TOM: So it's possible your mechanic got the tire size right, but he's confusing "maximum pressure" with "recommended pressure." The rating on the sidewall of the tire is maximum allowable pressure. That's the pressure after which the tire can explode. That doesn't mean you should use that much pressure, it just means "don't EVER use any more than that." It's also the pressure above which the tire wear increases dramatically, and handling can become dangerous.

RAY: Maximum pressure is like elevator capacity. Just because an elevator has a maximum capacity of 15 people doesn't mean you should ALWAYS have 15 people in there.

TOM: Especially if my brother's one of them, and he's just coming back from Nunzio's Sub Shop.

RAY: The other reason I doubt that 40 psi is correct is because you're driving a Buick. Buicks are known for their soft rides. And if you're driving on 40 psi, you've probably got swollen lumps on your head from hitting the roof every time you go over a bump. And that can't be what Buick intended.

TOM: Here's what I'd do, Doug. First call Buick Customer Service at (800) 521-7300 and ask for the acceptable sizes of replacement tires for your car. And ask if any of them have different recommended pressures. My guess is that all of them will call for 30 psi.

RAY: If your new tires are correctly sized, then go down to your local gas station and set all the pressures to 30 psi.

TOM: If your tires are not among the recommended sizes, go back to your tire dealer and ask him to give you a set that's appropriate for your car. Bring the information from Buick to show him, in case he hems and haws. And then have him set your pressure at 30 all around.

RAY: Then you'll be driving safely, Doug. And those lumps on your head will heal in a couple of weeks. Good luck.