The penny test is simple. Hold a penny so that you can see Abraham Lincoln's head. Now insert Lincoln's portrait, scalp-side down, into a groove in your tire tread. If your can still see the top of his head, it's time for new tires.
That works because the distance between the edge of a penny and the top of Lincoln's head is about 2/32 of an inch (in normal speech, that would be 1/16 of inch, but tire treads are measured in 32nds of an inch).
But now some experts advise a more conservative approach. Instead of Lincoln's head on a penny, use George Washington's head on a quarter. That's about 4/32, or 1/8, of an inch. In track tests conducted by the tire Web site TireRack.com, using a quarter instead of a penny resulted in 24 percent shorter wet-road stopping distances.
If you can see Washington's wig, your tires are as close as you'll want to get to being bald.
