New tires have approximately 10/32" of tread. Legal replacement is required at 2/32" — but performance degrades long before that.
At 4/32", wet-weather stopping distance increases by roughly 40% compared to new tires. At 3/32", you're skating on borrowed time in rain. Hydroplaning risk goes up sharply.
The penny test: insert a penny upside-down into the tread groove. If you can see all of Lincoln's head, tread is below 2/32" — illegal. More reliable is a tread depth gauge, available for $2–$3 at any parts store.
Replace at 4/32" for wet-weather safety. At 2/32" you're driving on tires that are legally OK but practically dangerous in any meaningful rain.
Tires wear unevenly. Front tires wear faster than rear on front-wheel-drive vehicles. Rotation every 5,000–7,000 miles equalizes wear and extends overall life.