Tire tread is vital to your safety. Why? Because the shallower a tire's tread, the less its ability to disperse water on a rainy day. And if it can't disperse water, then you run a serious risk of aquaplaning. So, what is tread depth?
It is simply the distance between the top of the tread to the bottom of the deepest tread groves. In the United States, this is always measured at 32nd of an inch with no exceptions!
The best and most accurate way to measure tread depth is with a Tire Tread Depth Gauge. If that is not readily available, a quick and easy trick is to use a Lincoln penny.
If Lincoln’s entire head is visible, that means that 2/32” of tread is still available, and that means that the tire is legally worn out, in most states.
The typical new tire used on automobiles is measured with 10/32” to 11/32” of tread depth. With winter/snow and light truck tires the tread is deeper because this depends on the tire’s tread type, which is either the Highway Rib, Highway All-Season, Off-Road All-Terrain or Off-Road Maximum Traction.
However, remember that the useable tread depth is measured by subtracting 2/32” from the tire’s new tread depth. So, on a tire that starts with a new 10/32” depth and has worn off 4/32” is now 50% worn.
The final 2/32" of a tire's tread depth isn't included in the tire’s tread calculation because the tire is considered already legally worn out with 2/32" of remaining tread depth.
Lincoln vs Washington test
Rather than using a Lincoln penny, a Washington quarter is more realistic, so say some tire aficionados. With a 2/32” of remaining tread on a Lincoln penny, the typical tire is already close to bald in rainy or snow conditions.
However, with a Washington quarter showing his head, it will show 4/32” remaining which is a more realistic tread depth, especially in rainy conditions.
A better method of measuring stopping distances is to actually measure rainy tire distances. Using a school bus for stopping distances, Good Morning America, got the following figures:
New Tires 10/32” tread – School bus on a wet road:
Stopping distance 195 feet
Worn Tires 4/32” tread – School bus on a wet road:
Stopping distance 290 feet
Bald Tires 2/32” tread – School bus on a wet road:
Stopping distance 379 feel
Obviously, the more worn a tire gets, the greater the stopping distances, especially at 4/32” to 2/32” tread range.
Tread depth guide
This handy tool is available at auto parts stores. There are many models available, but the cheapest ones are of a graduated probe design. All gauges should measure in both 32nds of an inch and millimeters.
Tread wear indicator bars
This one doesn’t cost anything because every tire has one, whether it is a performance, light truck, or medium commercial tire comes equipped with indicator bars (wear bars) embedded between the tread ribs at 2/32”. If the tread is flush with the wear bars, then it is time to replace the tires.
The tread wear indicators will indicate that your tires are hitting 2/32” and are thus still legal; however, it is probable that your tires will no longer save you from hydroplaning or lose control in rainy or snow conditions especially if you are going fast.
