The tire tread provides the gripping action and traction that prevent your vehicle from slipping, or sliding, especially when the road is wet or icy. In general, tires are not safe and should be replaced when the tread is work down to 1/16 of an inch.
Tires have built-in tread-wear indicators that let you know when it is time to replace your tires. These indicators are raised sections spaced intermittently in the bottom of the tread grooves. When they appear "even" with the outside of the tread, it is time to replace your tires.
Another method for checking tread depth is to place a penny in the tread with Lincoln's head upside down and facing you. If you can see the top of Lincoln's head, you are rady for new tires.