Like every other wear component on a vehicle (brakes, batteries, belts and wiper blades, etc.), tires must be eventually replaced because a combination of wear, damage or exposure to the elements will eventually take their toll.
Brake pads wear thin, batteries lose power, engine belts crack and wipers streak. When it comes to tires, a number of conditions or combinations of them will make replacement necessary.
Tread Wear – Tires provide traction through their tread rubber conforming to road surface irregularities as they roll into and out of contact with the pavement. However in addition to the traction, that interaction tears away microscopic amounts of rubber with every revolution. It’s important to note that typical tires roll about 36 million times over the course of their 45,000-mile life.
Dry roads don’t represent much of a challenge for the tire tread to remain in contact with the pavement because air is easy to push aside at any speed. However rain, slush and snow represent real 3-dimensional challenges for every tire to maintain contact with the pavement. When tire treads are worn too thin to accommodate wet and wintry driving conditions, longer stopping distances, reduced cornering traction and the tread’s ability to resist hydroplaning become concerns.
Tires are considered completely worn out when their remaining tread depth reaches 2/32nds-inch (1.6mm). Tires worn this far are unable to meet the challenges to deliver traction on wet, slushy or snow-covered roads.
Uneven Tread Wear – Vehicle misalignment and/or lack of periodic tire rotation can cause uneven tread wear that reduces the tire’s noise comfort, traction and driveability. While misalignment or omitted tire rotations may be the real cause of the irregular wear condition, drivers feel cheated anytime tire noise or driveability becomes objectionable before the treads wear out.
While easily corrected if discovered early, saw-toothed tread block wear due to under inflation or lack of tire rotation will eventually result in a reduction of noise comfort. Inboard/outboard tread wear caused by misaligned camber; or feathered lateral wear caused by misaligned toe-in/toe-out will prevent tires from contacting the road as originally designed by their engineers.
Road Hazards – City streets and highways are often strewn with accident debris and many exhibit pothole decay. This increases the risk of cut treads, punctured casings and impacted sidewalls that require inspection to determine if replacement is a safer option than repair.
Cuts and punctures that visually expose steel belts or casing plies is a reason for immediate replacement because those conditions will permit moisture and chemicals to weaken the bond between the rubber and the tires reinforcing cords. Multiple punctures in the same tire or a single puncture outside the tire’s repairable tread area require the tire to be replaced.
Sidewall bulges, impact breaks and rim cuts also require immediate tire replacement. They are a visual indication that the outside rubber compound has been damaged and has begun the process of loosing its bond to the tire’s underlying casing plies and belt package.
Tire Age – Tires are made with natural and synthetic rubber covering steel belts and fabric body plies. But like all rubber products their rubber will eventually harden and age.
Common current industry policies fall between vehicle manufacturer recommended 6 years of service (mounted, inflated and/or installed), to the tire manufacturer recommended 10 years from when the tire was built.
Tires warehoused properly (at moderate temperatures, away from direct sunlight and sources of ozone) are felt to have a shelf life of up to four years before beginning their 6 years of service.
It is recommended that tires that are more than 10 years old be replaced, even if they still appear to have adequate tread depth.
The wear created by thousands of repetitions and exposure to the elements
The industry standard used by the United States tire industry for measuring tire tread depths is in 1/32nds-inch increments (millimeters are used in countries observing metric standards).
Tires used on cars, vans, crossovers and light-duty light trucks are typically molded with a beginning tread depth of 10/32nds-inch, while tires used on heavy-duty trucks, commercial vans and tractor trailers are typically equipped with tires featuring deeper beginning tread depths.
While some consumers don’t consider their tires completely worn out until their tread has been completely warn away to the point of being bald; tires without grooves cannot maintain traction and control in foul weather and wintry conditions. Because of this, most countries (and most States) stipulate
U.S. Federal Safety Standards require all tire tread designs include tread-wear bars across their tread design when 2/32” (1.6mm) of tread depth remains. All tires are considered completely worn out when the tire’s tread design has worn even with the tread-wear bars.
This means a tire’s tread depth 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 groove. This is the value typically published by tire manufacturers.
Useable tire tread depth: Also measured from the top of the molded tread to the bottom of the deepest groove, however useable tire tread depth subtracts the last 2/32nd inch, where the wear bars would be equal to the tread design and the tire would be considered worn out.
For example, a tire beginning with a New Tire Tread Depth of 10/32nds-inch of tread depth is considered to have only offer 8/32nds-inch of Useable Tire Tread Depth.
The measurements to calculate tire warranty considerations are always based on the tires Useable Tire Tread Depth.
The U.S. practice of measuring tires in 1/32” increments was selected because it can be measured with a tire tread depth gauge. Because most passenger car and light truck tires have a Useable Tire Tread Depth of 8/32nds-inch, each 1/32” of wear represents 12.5% of most tire’s life, a useful value when determining wear and replacement warranty replacement costs.