Checking the depth of one's tire treads is easy to do with a penny. Slide the penny between the ribs of the tread on the tire to determine whether the tread has shrunk from its original size to 2/32 inch, which is the recommended threshold for replacement.
In many states, tire shops are required by state law to replace tires on cars that come in with a tread of less than 2/32 inch. The penny test works best when one turns the penny to point the head of President Lincoln downward toward the center of the tire. Replacement of the tire is recommended or required if his entire head remains visible, because that means that the tread is less than 2/32-inch deep.
It is important to check each tire and to check in a few places on each tire, as tread does not wear evenly from one tire to the next or at all points on the same tire.
Another way to test tread depth is to look for tread bars, which manufacturers embed at 2/32 inch. If the tread bar shows up, it is time for a new tire.
Inexpensive tread depth gauges are also available at auto part stores and show the exact measurement of tread that is left on the tire.