Traditional advice says that when your tire tread is worn down to 2/32 of an inch (and that's the law in some states) or when the tread wear indicators are showing, then it's time for new tires.
With many tires, though, drivers will experience a significant loss of traction and braking ability in rain and snow before then. Because tires wear gradually, and many vehicle owners pay little or no attention to tires, the loss of traction may not become apparent until the vehicle skids instead of stopping on a dime, as it used to.
New tires typically have from 10/32 to 11/32 of an inch of tread depth when they're new. The deep tread, plus grooves and slits cut into the sides of the tread, allow water and snow to escape from under the tire so it can maintain adequate grip. As the tread wears and the grooves and slits become shallower, more moisture remains trapped under the tire, which then rides on a slippery surface of water ("hydroplaning") or snow instead of "biting" the pavement.
The result is longer stopping distances, more wheel spin in acceleration and less grip in turns.
When this slipping and sliding starts to occur — and how severe it is — will vary by tire. With some tires, it could come when there's still, say, 5/32 of tread depth left, which would seem to be more than enough to avoid buying new tires. Some tires, though, simply have better wet-weather traction than others and will maintain it for more miles.
Mechanics can inspect tires for unusual or excessive wear, measure tread depth with a gauge and advise how much life is left. Tread depth gauges are available at parts stores for do-it-yourselfers, plus there's always the penny test: Insert a Lincoln penny (head first) into a tread groove; if you can see the top of Abe's head, you need new tires.
