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 when to buy new tires
Tires degrade over time due to environmental exposure, regardless of how much tread is left. How to Buy Tires: A Step-by-Step Guide - US News Cars Tread depth is the primary indicator of a tire's health