Note: Also check the hardness of the rubber. Sometimes a tire can have plenty of thread left but because the tire is old, or had been subject to extreem weather the tire looses it’s gripiness.

Here's the three-step "penny test:"

1. Take a penny and point Abe's head down towards the tire thread.
2. Look for the point on your tire where the tread appears the lowest (most worn out). Put Lincoln's head into one of the grooves on the tread.
3. If any part of President Lincoln's head is obscured by the tread, you're driving with the legal and safe amount of tread. If you can see above his head, where it says "In God We Trust," take a hint: you're ready for a new tire.

Why worry about tread depth? If your tread gets below 2/32 of an inch, your car's ability to grip the road in adverse conditions is greatly reduced.

Insufficient tread is not only dangerous; in many states it is illegal (legal tread depth is 2/32 of an inch - the exact distance from the tip of Abe's head to the rim of a penny).