Tire Repleacement Guidelines
One of the most common tests to tell if your tread is no longer providing your vehicle with enough safety is to take the penny test. Take a U.S. penny and place the top of President Lincoln's head between a section of tread on your tire. If your tread covers over the tip of his head, your tires are still good. If it does not, your tire has used up its useful life.
This test should be conducted every 12 inches or so around the tire, making sure to test the outside, middle, and inner portion of the tread. Sometimes an alignment problem on a vehicle can cause tires to wear unevenly, so it is important to identify if certain areas are wearing faster than others. Uneven tire wear can cause a tire to need replacement long before it would normally.
Cracks and Bubbles
A sign that a tire may be destined for failure is deep cracks and/or bubbles on the tread or the sidewall. There are several layers that go into a tire, so minor cracking may not always be a serious condition, although any amount of deterioration should be cause for concern.