Tread depth is the primary indicator of a tire's health. New tires typically start with of tread.

Many experts, including Tire Rack and SimpleTire , recommend replacing tires at 4/32" , especially if you drive in wet or snowy conditions where traction drops significantly below this point. Quick DIY Tests

Insert a penny upside down into the tread. If you can see the top of Lincoln's head, your tread is below 2/32" and the tire is unsafe.