A tread depth of 6/32 inch is not ideal, since it means the tire has worn half the new tire tread depth of 10/32 to 12/32 inch. However, a tire with 6/32 inch of tread depth is still safe for driving in rain and snow.
Tread depth can be measured with American coins. Put a penny into several grooves across the tire. If part of Lincoln's head is covered, over 2/32 inch of tread remains. If Washington's head on a quarter is always covered, then 4/32 inch of tread is left. If the Lincoln Memorial on a penny is always covered, there's 6/32 inch of remaining tread depth.
Place the coins across the tread every 15 inches around the tire. This reveals uneven wear that mechanical or service conditions may cause.
A tire in most states is legally worn out at 2/32 inch of tread depth. In North America, this level is indicated with wear bars that run across the tread pattern.
A worn tire can't perform as well in rain and snow. At 2/32 inch of tread depth, a car may hydroplane at highway speeds and has virtually no traction in the snow. For rainy conditions, drivers should consider replacing tires with 4/32 inch of tread depth, as the grooves need to be deep enough to allow rain to escape and prevent hydroplaning. For snowy conditions, a tread depth of 5/32 inch is needed to compress the snow in the grooves and release it as the tire rolls.