Checking Your Tires' Treads
There are several simple ways to check tire tread depth. The first way is to measure the tread depth with a gauge. The second, and probably the most practical way to check involves using a penny that is inserted into the grooves of the tread. Checking a tire's treads periodically ensure the safety and effectiveness of your tires. Tire wear is natural, and excessive wear can lead to considerable loss of traction, especially on wet surfaces.
When using a tread depth gauge, tires need to have at least 1/16" of tread or more as this is the minimum allowed by law. By using a penny as reference, one can insert it into the tire's tread groove with the face of Lincoln showing, with his head upside-down. If Lincoln's head is clearly visible in its entirety, you should seriously consider replacing the tire. If you see a wear bar across the width of the tread while facing it, it's time to replace the tire. If your tires contain steel treads, and you can see the metal showing through the rubber material, this is a severe tire problem and you must have the tire replaced immediately. Driving on tires with visible steel treads is a major safety risk.
Generally, it's best to replace tires in sets of four; this keeps the wear and tear even across all of your tires, and prevents problems that can appear with poor alignment and poor balance. If your car's tires show signs of abnormal or unequal wear, have this looked into by a professional technician. Excessive wear on both outer edges generally indicates under-inflation. Excessive wear in the center of the tread generally indicates over-inflation. Cupping or dipping of certain tread sections may indicate worn suspension parts or a wheel balance problem. Saw-toothed or feathered tread edges may indicate wheel misalignment.
Here's a great money saving tip. Most auto shops will charge between $30 and $50 to do a rotate-and-balance job. However, you can usually get a tire dealer to do this for free when purchasing tires. If your car needs alignment or suspension work, and you're buying a set of tires, have this work done before you drive off with your new tires. Most installers will do a rotate-and-balance for free when you purchase the tires from them.
Sometimes, prices may tempt you to purchase used tires instead of new ones. Be extremely careful when purchasing used tires. Be on the lookout for a practice called "grooving," which compromises your safety and is illegal in the US. Some unscrupulous tire dealers will take used tires with very worn treads, and use a "grooving" machine (or even a drill) to shave away rubber, making cuts along the tread pattern. This gives the treads the appearance of being deeper and less worn. It also makes the tire very dangerous and increases the risk of blowout and other tire failure. When inspecting used tires, look for a uniform color. If the grooves in the treads appear much darker in color than the rest of the tire, or if the treads are jagged, it's possible that the tire has been illegally "grooved."
Taking the big picture approach to protecting your tire investment will reap rewards for many miles to come.
The American Automobile Association: Tire Tread Inspection
Reviews Online: Checking Your Tires