THE PENNY TEST
Checking for your tire's tread depth is a simple task that you can do yourself. The standard test to check for tread depth is the penny test. It is an easy test to do and is very accurate. To do this, simply use a penny, and put it between the tread blocks of the tire with Lincoln's head upside down. If you are not able to see the top of Lincoln's head, then that is a good sign, meaning you still have more than 2/32" of the tread remaining and they are still safe to drive on. If you can see the top of Lincoln's head, then that means that the tread is worn down and it is time to shop for new tires to ensure safe driving.
The penny test is accurate and has been used for ages, but there are other factors to keep in mind. The standard car tire has about twenty square inches of total footprint surface touching the road, with about 1/3" of tread depth with new tires. Most of this so called "footprint" is composed of rubber which grips the road, but the rest is the space of the grooves that create the tread design. The performance of both the actual rubber of the tires and the grooves affect tread wear. Similar vehicles with different tread depths will yield different results when breaking. The vehicle with a tread depth of 4/32" will stop about 100 feet sooner than the vehicle with 2/32" tread depth. That is a big difference, and on slippery roads, that difference can be even greater.
Another factor is how well tread depth contributes to the design of the tire so that it performs at its best. Liquid is more dense than the air that fills the tires, so going about 65 MPH allows for the air to be compressed and easily moved out of the way. However, with liquids, it is not as easy. In poor weather conditions, such as a rainstorm, when water collects on the road, many factors come into play, including depth of the water on the road, speed of the vehicle, vehicle weight and tread design on the tires, and these factors determine whether your vehicle will hydroplane and in response, how quickly the vehicle can be stopped.
Traction is what keeps the car on the road, and traction between tires and the road is the largest factor in the driver's ability to keep control of the vehicle.
Most new tires have indicator bars built in that measure tread wear. The bars are nearly invisible when the tires are brand new, and as the tread gradually wears down, the bars become more apparent. They are flat rubber bars, perpendicular to the tread. If more than one is visible, then the tread is low, and you should consider getting your tires rotated or replaced.
Completing the penny test is a good way to check whether your tires are worn, but there are other important things to keep in mind as well. Tread design and traction play huge roles in safe driving and tire life.