Originally Posted by GaryS
Snow tires need minimum 6/32" tread. There are usually wear bars at that height between the treads that make it obvious.
The wear bars indicate whether the tire is safe to drive on, but they don't indicate whether the tire will still do the snow job. I think a snow tire's grip is a function of the sharp edges on the hundreds of sipes in the tread blocks -- at the micro level, those edges slice the ice just like a skate blade. Once those sipe edges are rounded, the tire's icy road performance will be gone, regardless of how much tread is left.