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. You can also test your tread with a Lincoln penny. Simply turn the penny so Lincoln’s head is pointing down and insert it into the tread. If the tread doesn’t cover Lincoln’s head, it’s time to replace your tires.