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Tread depth is a vertical measurement between the top of the tread rubber to the bottom of the tire's deepest grooves. In the United States, tread depth is measured in 32nds of an inch. Because it is difficult to accurately measure tread depth with a ruler, tread depth is best measured with a tire tread depth gauge. When tires have neared the end of their life, a U.S. Lincoln penny can also be used to confirm the tire's tread depth. If Lincoln's entire head is visible, the tire is worn to approximately 2/32" and is considered legally worn out in most States.
Average new tires used on cars typically start with 10/32" to 11/32" of original tread depth. Dedicated winter / snow tires and light truck tires typically are deeper (for light truck tires, how much deeper depends on the tire's tread type ... Highway Rib, Highway All Season, Off Road All Terrain or Off Road Maximum Traction).
As mentioned above, tires are legally "worn out" In most States when they reach 2/32" of remaining tread depth. For example, a typical tire that starts with 10/32" of original tread depth has only 8/32" of useable tread depth. Its useable tread depth is calculated by subtracting a worn out tire's 2/32" from the new tire's original depth of 10/32". The final 2/32" of a tire's tread depth isn't part of the equation when it comes to calculating tread depth percentages because the tire is already legally worn out with just 2/32" of remaining tread depth.
Useable tread depth is calculated by subtracting 2/32" from the tires new tread depth. Then usable tread depth is compared to remaining tread depth in order to calculate tread wear percentages. For example, a tire that started with 10/32" of original tread depth and has worn off 4/32" (down to 6/32" of remaining tread depth) is 50% worn.
10/32" original tread depth
-2/32" legally worn out tread depth
8/32" of useable tread depth.
8/32" useable tread depth
-4/32" measured tread depth worn away
4/32" of remaining tread depth (50% of 8/32")
A tire that starts with 10/32" of original tread depth has 12.5% wear for every 1/32" that is worn away, and a tire that starts with 12/32" " of original tread depth, has 10% wear for every 1/32" that is worn away, etc.
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