If could see the top of Lincoln's head when you inverted a penny (Lincoln's head upside down), and stuck it down into the tread, then the tire was completely worn. To effectively measure tire tread depth, you first need to know how to position the coin. Place a penny with Lincoln's head upside down and facing you into the. The Penny Test · Place your penny into a tread groove upside down. In other words, your penny should be going into the tread headfirst! · Examine the part of the. You can find tire tread depth gauges at your local auto parts store. Those changes are up to federal and state governments. In addition to testing your tread once or twice annually, pay attention to the way your vehicle handles. Tire Rack, which studies and sells tires, says that the penny test is outdated and that drivers should use a quarter instead. Also, having insufficient tread is considered illegal in many testing tire tread with a coin. Tire pressure factors heavily into vehicle handling and can, in some cases, change the depth reading of tire treads. All gauges should measure in both 32nds of an inch and millimeters.