Traditional advice says that when your tire tread is worn to the point where the groove is only 2/32 of an inch
(and that’s the tread depth law in some states) or when the tread wear indicator bars show, it’s time to put new tires on your car.
With many tires, however, drivers will experience a significant loss of traction and safe braking ability in rain and snow by that point. Since tires wear gradually and many vehicle owners do not regularly check their tire tread depth or uneven wear, the loss of traction may only become apparent when the vehicle skids instead of stopping on a dime as it used to.
New tires typically have a tread depth of 10/32 to 11/32 of an inch when new. The deep tread, along with the grooves and sipes cut into the sides of the tread, allow water and snow to escape from under the tire to maintain adequate grip. As the tread wears down and the grooves and sipes become shallower, more moisture remains trapped under the tire. The tire then rides on a slippery surface of water (“hydroplaning”) or snow instead of “biting” into the pavement.
The result is longer stopping distances, more wheel spin during acceleration, and less grip in turns.
When this slipping and sliding starts to occur – and how severe this lack of traction is – will vary by tire design and could happen well before it seems like you have bald tires that need replacing. With some tires, the safety loss could occur when there is, say, 5/32 of an inch of tread depth left, which seems more than enough to avoid buying new tires. Some tires, however, simply have better wet and snow traction than others and will maintain it with less depth over more miles.
Mechanics can inspect tires for unusual or excessive wear, measure tread depth with a gauge, and indicate the remaining tire life. Tread depth gauges to check worn tires are available at auto parts stores for DIYers, plus there’s always the penny test: Insert a Lincoln-head penny (the top of the head should go in head first) into a tread groove; if you can see the top of Honest Abe’s head, you need new tires.