The "penny test" is no longer good enough to tell whether your tires have enough tread left, according to The Tire Track, a popular online retailer, and Consumer Reports. For decades, the "penny test" has been a rule of thumb: If you can see the top of Lincoln's head when you hold a penny upside down in a tire tread groove, you've got less than 2/32-inch of tread left and it's time to replace your tires. In July 2007, however, tests conducted by The Tire Rack showed that braking distances on wet pavement increased dramatically when the tires' tread depth was that shallow, posing a potential safety hazard in panic-stop situations.
Both Consumer Reports and The Tire rack now recommend using the "quarter test" as an easy way to measure tread depth. Instead of using a penny, place a quarter upside-down in your tire's tread, if the top of George Washington's head is visible, the tread depth is 4/32-inch, and it's time to start considering new rubber for your ride.