Most American tires manufactured as of 2010 have wear indicators---or bars---built into them, designed to warn owners that their tires have become dangerously worn out and need replacing. Before your tires wear down to a thread, check tread depth with a tire gauge or even a coin to determine the extent of the wear. "According to most states' laws, tires are legally worn out when they have worn down to 2/32" of remaining tread depth," says Tire Rack.com.
Measure your tires' tread depth with a tire gauge---available at auto parts stores. Typical gauges measure tread depth up to 32nds of an inch. Some gauges measure tread depth in millimetres, however; check your gauge type before you start.
Press the tire gauge onto a hard surface until it reads "0," then push the measuring scale into the gauge until it doesn't go in anymore. Insert the gauge's probe into an inner or outer tire groove, and push on the base. Hold the barrel of the gauge---not the probe---and carefully lift it off the tire. Look at the gauge to obtain your tread depth reading.
Continue measuring tread depth in the same tire by inserting the probe in different areas---alternating in both inner and outer tread grooves---about 15 inches apart along the circumference of the tire; record each measurement carefully. Obtain an average by adding up the total of the measurements and dividing by the number of measurements you took. Repeat the measuring process for all four tires on your car.
Use a penny to measure your tyre's tread depth---instead of using a tire gauge---if the tread looks worn down to the last 32nds of an inch. Turn the penny so Lincoln's image faces you and the top of his head points downward. Insert the penny into a few different tread grooves at least 15 inches apart from each other, and observe Lincoln's head. If the tread always covers it, your tire still has more than 2/32 inches of tread.
Turn the penny around and insert it into different tread grooves around the circumference of the tire at least 15 inches apart to see where Lincoln Memorial lands. If the Memorial is always buried, your tire has more than 6/32 inches of tread left and your tire is in good shape.
Place a quarter into several inner and outer grooves of your tire with the top of Washington's head pointing downward, moving across the circumference of the tire at least 15 inches apart from the other measurements to see whether the tread consistently covers Washington's head. If so, your tire has more than 4/32 inches of depth left and you have no need to replace it yet.
Determine the percentage of tread wear by comparing your average measurements with the original tire tread depth when they were new. Placing a tire gauge or a coin in several areas of your tire will help you to locate any uneven tire wear.
Avoid placing the probe of the tire gauge directly on tread wear indicators or bars.