Once tire tread is worn down to 2/32 of an inch, it’s time to buy tires. A lot of tires have bars in the tread that indicate when the tire is so worn that it should be replaced. Regardless of what type of tires you have, you can easily check the tread depth yourself: Put a penny in the tread with Lincoln’s head upside down; if the top of Lincoln’s head can be seen, the tire is too worn to continue driving on.