The bare minimum of tread a tire can legally operate with is 1/16 (2/32) inch. Many new tires have "wear bars" to indicate when the tire needs to be replaced. Wear bars are thin band inside a tire's tread at the exact legal minimum limit. When the tread has worn equal to the wear bars, the tread has reached 2/32 inch, and it is time to replace the tire. If your tires do not have wear bars, then the "penny test" is the best way to determine your tread level.
Things You'll Need
Grasp half of the penny between your thumb and index finger. Be sure you can still clearly see the top of Abraham Lincoln's head.
Find a groove on the tire where the tread appears to be worn the most. Stick the top side of Lincoln's head into the groove.
Observe the penny. If any part of Lincoln's head is covered by the tread, you are still riding within the legal limit. If, however, you can clearly see Lincoln's entire head and any part of the phrase "In God We Trust," your tire is in need of replacement. You have worn the tire past the legal limit.
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