Merle King works on balancing a tire at Bagsby Tractor and Truck repair shop in Spring Hill, Tenn., on Oct. 1, 2009.

AP Photo/Josh Anderson

Replacing Car Tires

With some car parts, you simply don't know it's time to replace them until they actually break. But with regular visual inspection, you'll know it's time to replace your tires when there's no longer adequate tread to help your car stay safely in contact with the road surface. You can perform the "penny test" to see if the tread is deep enough. Simply take a penny and insert it (upside-down) into the tire's tread groove. If the top of Abraham Lincoln's head is visible, then the tread is worn out. If you have a ruler handy, that's 1/16 of an inch (1.59 millimeters).

And of course, if you spot deformities such as deep cracks or bubbles anywhere on the tire, you should have it checked out by a tire dealer or auto parts and service center. Replacement may be in order. The tire shop has access to a wealth of car part information that can make your job of choosing which set of tires to select a lot easier.

When you need to replace a worn-out tire, chances are you'll want to replace more than one. Tires are usually replaced in sets of two or four. That's because having a difference in the traction capabilities at each corner can lead to a dangerous loss of control.

For safety and best performance, it's recommended that all tires on your vehicle are the same size, speed rating, and type -- either radial or non-radial. The exception would be on some sports cars that have bigger tires (diameter and width) on one axle than they do on the other.

Even on a front-wheel drive car, if you can only replace two tires, they should go on the rear axle: The greater tread depth in the back helps maintain control in wet and slippery conditions. It's highly recommended by tire experts that you never replace just one tire at a time. If it's unavoidable, however, you should pair the new tire with the existing tire that has the least tread wear [source: Rubber Manufacturers Association].

Your tires are just one example of this "well-worn" truth: For greater car part longevity, nothing beats a schedule of regular monitoring and maintenance. Extending auto part longevity, including that of your car's tires, is a smart way to save money and avoid sending used-up materials to the landfill prematurely.

For more about car tires and other auto part information, follow the links on the next page.