The Lincoln Penny Test has become the tried-and-true method of determining if it is time to purchase new tires for your vehicle.
Take a penny and pinch between your thumb and forefinger at the bottom of the coin, making sure Lincoln's head is the top of the coin.
Turning the coin upside down, making sure Lincoln's head is in one of the grooves on the tread. Select a tread on the tire where the tread appears the lowest.
If any part of Abe Lincoln's head is obscured by the tread, you're driving at the legal and safest amount of tread. If you can see above his head, the thread may be below the legal limit and it may be time to purchase new tires.