The tire industry standard used for measuring tire tread depths in the United States is in 1/32nd-inch increments (millimeters are used in countries observing metric standards).
New tires fitted to cars, vans, crossovers and light-duty light trucks are typically molded with a beginning tread depth of 10/32nd- to 12/32nd-inch. Tires used on heavy-duty trucks, commercial vans and tractor trailers are often equipped with deeper treaded tires.
Most countries, states and tire manufacturers stipulate tires are worn out when their tread reaches 2/32nds-inch of remaining tread depth measured in their circumferential grooves. While completely bald tires (like racing slicks) can deliver traction on dry roads, bald tires are unable to meet the additional challenges associated with driving on wet, slushy or snow covered roads.
U.S. Federal Safety Standards require tire tread patterns include 2/32nd-inch treadwear indicators across their tread. Tires are considered worn out when the tire’s tread pattern has worn even with the treadwear indicators.
Tire tread depths can be described in different ways:
New tire tread depth: Is measured from the top of the molded tread to the bottom of the deepest circumferential groove. This is the tread depth value typically published by tire manufacturers.
Useable tire tread depth: used only for tire warranty percentage of wear calculations. This value is also measured from the top of the molded tread to the bottom of the deepest groove. However useable tire tread depth subtracts the last 2/32nds of an inch, where the tread pattern has worn down to the treadwear indicators and the tire is considered worn out.
Example: A tire beginning with a New Tire Tread Depth of 10/32nds-inch is considered to have only 8/32nds-inch of Useable Tire Tread Depth.
Prorated tire warranty tread depth percentage calculations are based on the tires Useable Tire Tread Depth.
Since most passenger car and light truck tires have a Useable Tire Tread Depth of 8/32nds- to 10/32nd-inch, each 1/32nd-inch increment of wear represents 12.5% to 10% of the tire’s life, a useful value when determining percentage of wear the customer has received used as a basis for prorating the replacement warranty costs.