By Bill Howard

They're all round, almost always black and they seem to last forever if you don't hit a pothole. Yet tires make a big, if unexciting, difference in your car's performance and safety, especially in the rain, slush and snow of the winter months. For that reason, buying a second set of tires for winter driving may be a good investment.

Ironically, what makes the tires on your car last so long may be a reason why you should think about a second set. The black rubber in different tires only looks the same. The compounds are markedly different depending on the type of tire. Original equipment manufacturer, or OEM, tires that came with your car were most likely biased toward affordability, fuel economy and long tread life. Those attributes may also bias the tire away from best performance in the winter. The same often holds for tires advertised as being long-life (more than 50,000 miles).

You'll encounter several types of tires among mainstream cars and SUVs including:

All-season. They're the most common tire sold, or put on new cars, and are adequate for light snow. They'll have an M+S stamp on the sidewall that stands for mud & slush.

Touring. They're meant for a quiet ride, are not meant for use on snow or ice and handle better than all-seasons. Sometimes they're called summer tires.

All-season touring. As the name implies, they provide reasonable performance and handling even in light snow, trading off a bit of both.

Performance. They're great in the summer but the sticky rubber compound has a relatively short tread life and becomes unpliable in cold weather. If you have performance summer tires, you really should take them off in winter.

Winter tire. The tread is more pronounced, the rubber doesn't get hard below freezing and they're designed for long, snowy winters. There'll be a snowflake-and-mountain symbol on the sidewall.

Within these categories you'll find gradations, such as a winter performance tire, meaning it trades off maximum snow-and-ice traction for better ride and handling on all the days when the pavement is dry.

How to Choose the Right Tires

When you talk tires, you need to decide what's most important. Is it tread life, handling or ride comfort? (And no fair saying "all of them.")

The government has a tire rating system (the Uniform Tire Quality Grade Standards) that may help you pick the most proper tire type, brand and model.

Tread wear rates a tire's durability relative to an industry standard tire, whose rating is 100. A tire rated at 200 should last twice as long as the benchmark tire, but critics say the number is subject to fudging by automakers.

Traction is rated as AA, A, B or C. AA is best.

Temperature grades relate to the sustained speeds the tires can handle. C for 85-100 mph, B for 100-115 mph, and A for speeds over 115 mph. The higher the speed rating, the higher-performance the tire is likely to be.

Winter Tires: What to Think About

If you do consider a second set of tires for the winter months, consider these tips:

Buy four tires, not two. The handling characteristics of winter, summer and all-season tires are vastly different. The Tire Rack, an Indiana direct reseller of tires and wheels, runs a snow tire handling program each year for editors on a hockey rink (you drive very slowly in there) and an outdoor test track. Winter tires were markedly more responsive on ice. With a combination of snow and summer tires on a dry track, cars became unstable performing emergency lane changes. The car's skittishness was noticeable.

Get four snow tires. If you do opt for just two, and you shouldn't, the snow tires always go on the back, even with front drive cars. If front-only snow tires grip too well while braking, you risk the back and front ends swapping place as you spin.

Get wheels (rims) and tires. The cost of mounting and dismounting snow tires will quickly outstrip the cost of steel wheels for the snow tires. Plus, you don't place at risk costly alloy wheels that may corrode faster in winter and are more likely to be damaged by potholes. Some steel wheels look a lot like alloy wheels now; you don't have to buy ugly wheels.

Downsize wheels by 1 inch. If you have sporty tires and wheels, you often can decrease the size of the wheels by 1-inch (for example, replace 18-inch tires with 17-inch) and order tires with sidewalls ½-inch more (top and bottom) so the diameter is the same. The increased cushion protects against pothole damage, too. Tire dealers all have fitment charts that show acceptable sizes.

The most important thing to remember about tires is to know when to replace them. Tires come with about 10/32-inch of tread depth and must be replaced (it's required) when it's down to 2/32-inch, or when you stick a Lincoln's head penny head down in the tread and you can still see the top of Abe's head. You really should think about replacing at 4/32-inch tread depth: Stick a quarter head down in the tread; if Washington's head just disappears, that's 4/32-inch.

Bio: Bill Howard is an experienced automotive writer and the editor of TechnoRide.com, the car technology site produced by Ziff Davis. Previously, he was PC Magazine's executive editor. Howard is also vice president of the International Motor Press Association.