Astonishingly, of the 800 million tires that roll daily to various destinations, 80 million are legally bald. Many more are underinflated or the wrong size, endangering lives every time they hit the road. Tires should be replaced according to when the manufacturer's warranty dictates, typically between 20,000 and 40,000 miles. Each car has its own "lifestyle," affecting the proper selection of wheels, says Dave Finkelstein, auto tech columnist at the St. Louis Post-Dispatch and host of 97.1 FM's Auto Talk. "The biggest mistake people make is tossing their keys to the mechanic and letting them decide what to do," Finkelstein says. "You need to sit down with the service provider and explain how you use your car. Do you do a lot of highway driving? Do you stop and start a lot? Do you drive in snowy areas? These questions help the technician suggest which tires are best for you." All-season tires work well in dry and wet weather and can handle a certain amount of snow. Performance tires, often found on sports cars and sport editions, provide more grip. They tend to be more expensive and can wear down faster. All-season performance is a new breed that meets the needs of high-performance driving, but they also handle rain and some snow. Snow tires are constructed to grip snowy surfaces and to stay pliable in the cold. Improper sizing can lead to tire failure, but it can also undermine an automobile's overall performance. The manufacturer's tire selection dictates proper handling and braking. "Your car is calibrated by the type of tire designed for your car," says Dan Zielinski, vice president of communications for the Rubber Manufacturers Association. "You could think you are driving 70 miles an hour when you really aren't. It can change your odometer reading. You can purchase a tire that is too large and scrape the wheel cover until it will fail. So many things are based on its size." To ensure proper size, a sticker inside your driver's side door or your owner's manual will list the proper size and inflation of the tires on your car. When purchasing tires, it's important to note that there are far more important factors that should influence your purchase than price. Tires have a relatively short shelf life, says Finkelstein. "When they are too old they get cracks in them that develop into splits. It can become a serious safety issue." If purchasing tires online, carefully review the return policy. How much is shipping and handling? Who pays the shipping costs for returns? If you are required to pay, that may cancel out any special savings. There are also several measures that should be taken for basic tire maintenance: Check air pressure: It should be checked monthly. Underinflation causes unnecessary tire stress, irregular wear, and loss of control. Air pressure must be checked with a gauge. The right air pressure cannot be judged on sight. Most tire shops will check your pressure free of charge. Check alignment: Hitting a pothole can easily throw off wheel alignment, which can cause damage to your tires. Periodically, have your alignment checked by a mechanic. Rotate: Tires should be rotated every 6,000 to 7,000 miles for more uniform wear on wheels. Check treading: Always examine tires for damage and or advance wear, which can affect the tire's traction, particularly during adverse weather conditions. Tires must be replaced when worn to one-sixteenth of an inch or more. "Tires are the only part of the vehicle that touch the road," says Zielinski. "They shouldn't be treated as a haphazard. [The wrong tires] are equivalent to buying the wrong shoes." For more information on tire purchases and maintenance, visit the Rubber Manufacturers Association on the Web at www .rma.org or The Tire Rack at www.tirerack.com.