Pressure is on in tire debate- Let the maker be your guide

Dear Tom and Ray: When looking for the correct tire pressure, you say to check the recommendation by the manufacturer of the car, not the manufacturer of the tire. But how does the car manufacturer know what kind of tires I bought? I have no idea what kind of tires came with a '98 Volvo, but I'm pretty sure I haven't bought that type. Wouldn't the tire manufacturer know better what the pressure should be for its specific tire? Please clarify. I get different answers from different mechanics.

Thomas

Tom: Listen to the car manufacturer, Thomas.

Ray: When a manufacturer designs a car, it designs it for a certain-size tire. And even when you replace your tires later on, the tire SIZE should be the same. That's what the pressure recommendation is based on.

Tom: So, let's say your 1998 Volvo came with 195/60R15s. That means that Volvo designed the car for a tire with a tread that's 195 mm wide, an aspect ratio (the height of the sidewall) of 60 percent of the tread width, a radial design (the R), and a wheel size of 15 inches. When Volvo recommends a tire pressure, it's designed to give you the best balance of safety, handling, comfort and fuel economy -- for that car, with that size tire.

Ray: Does it matter if you replace your Michelins with Goodyears? Or if you switch from an all-season to a summer tire, or a tire with a different tread pattern? Not really, as long as the tire's basic specifications are the same. Presumably, when you replaced the tires on your Volvo, the mechanic or tire salesman knew enough to sell you correctly sized tires.

Tom: Now, there is a little wiggle room. If you get a slightly wider tire, or a slightly thinner tire -- say 10 or even 20 mm in either direction -- the carmaker's recommendation is still going to be close enough. In fact, if you buy winter tires, it's often advisable to get a slightly narrower tire to help cut through the snow.

Ray: But if you go way off the ranch and replace a 195/75 tire with a 235/50 tire, for example, then you're on your own. Not only will extra-wide tires put additional pressure on your bearings and steering components, but the correct pressure is now a matter of guesswork.

Tom: And by the way, you should never use (or exceed) the pressure number listed on the sidewall of the tire. That's not the recommended tire pressure. That's the maximum pressure that the tire can take before it's in danger of exploding. If you fill your tires to that amount of pressure, not only will your car handle like a basketball, but your head will bounce off the ceiling every time you hit a bump.

Ray: So your best bet is to stick to, or very close to, the car manufacturer's size recommendation when replacing your tires. If you do that, the manufacturer's pressure recommendation will always be a good guideline. That means if the local tire shop says, "We don't have your size in stock, but these bulldozer tires'll be even better," go somewhere else.