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If you live where the snow truly heaps up, or if you face a sea of mud each spring, you may need lugged mud-and-snow tires on the driving wheels for part of the year. Remember, though, that they will only outperform all-season tires when the snow is loose and deep or "when the world is mud-luscious." At other times, they will howl on the highway, wearing rapidly to uselessness.

Profile

And what of this wide tire craze? Is lower and wider really better? Let's look at what low-profile tires do, so you can judge whether they're right for you.

Contrary to conventional wisdom, wide tires were not developed to improve traction by increasing the size of the contact patch. An interplay between the weight of the vehicle and the diameter and flexibility of the tire determines that. If you compared the contact patches on two tires that were identical except for profile, the areas would be about the same. However, the shape of a wide tire's contact patch is quite different from that of a narrow one. Viewed in the direction of travel, a high-profile tire's contact patch is longer than it is wide, while a low-profile tire's is shorter than it is wide. The result is that a car with low-profile tires handles differently.

Low-profile tires can develop more cornering force because their wider contact patch stays more firmly planted when turning. Also, because the tire's sidewalls are shorter and therefore deflect less under side load, the car's steering is more responsive. Other advantages: Because the tire makes up less of the overall wheel-tire diameter, the wheel can be larger. This allows bigger brakes and makes suspension layout easier. All these attributes are very important to a racing driver.

On the other hand, a number of everyday concerns argue against wide-profile tires for general use. First, a car designed for narrow tires will be less stable in a straight line when fitted with wide tires. Without the longitu dinal contact patch, there is less force to center the steering wheel and encourage the car to go straight. What's more, wide tires generally don't ride as well as narrow ones because of their short sidewalls and their increased capacity to find bumps. Add to that the short sidewalls' vulnerability to damage from bumps.

Unless you're particularly interested in enhancing your car's ability to handle curvy roads, there's probably little reason to use a tire with an aspect ratio lower than 70 series. (See the sidebar on tire sizing for an explanation of aspect ratio.) Most cars come with 75- or 80-series tires, though 35-series, 17" rubber comes stock on the back of some Corvettes.

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