Tire Safety: Tire Tread Depth Using a Penny or a Quarter

Tread Depth, Tire Safety and the Difference of 24 Cents

A head-to-head challenge in the tire industry currently in­volves two of the nation’s greatest presidents. The debate is over a mere 2/32nds of an inch, but safety is held as most important on both sides of the coin.
Tire Rack officials suggest the traditional penny test needs to be replaced with a quarter test, since a tire's performance is substantially different with 2/32nds vs. 4/32nds of remaining tread.
Like the fate of the nation they led, it all rests on their heads.

For decades, a quick test for tire treadwear has consisted of placing a penny on edge in a tire’s grooves. If the top of Abe Lincoln’s head remains visible, the tread is less than 2/32nds of an inch, meaning it was time for new tires. In fact, tread depth below 2/32nds makes those tires “illegal” in some states. Even without a state law, most all industry players acknowledge the danger of riding on the worn-out tires.

It could be chalked up to inflation, but one of America’s leading high-performance tire and wheel distributors believes the penny needs to be replaced with a quarter. Tire Rack officials are promoting using the quarter’s inverted George Washing­ton to show that less than 4/32nds remain if the top of his powdered hair touches the tread.

This magazine has often called on the industry to adopt a strict 4/32nds limit on tread depths, quite similar to an ongoing campaign in Europe being pressed by both Conti­nental AG and consumer-focused TyreSafe.

Doubling the amount of remaining tread as a signal to replace tires makes a big difference in the distance required to stop a vehicle on a rain-slicked roadway, according to Woody Rogers, Tire Rack’s tire product information specialist.

Tests conducted by Tire Rack provide compelling results that favor the switch.

Using brand new tires with 10/32nds tread, worn tires at 4/32nds and legally worn-out tires at 2/32nds, the test consisted of 70 mph runs with a passenger car and pickup truck. Each run ended with a panic stop on asphalt covered by 0.05 inches of water, equivalent to a moderate rain shower.

With new tires, the car traveled 195 feet before coming to a stop; with worn tires (4/32nds), it went 290 feet. The same vehicle with worn-out 2/32nds tires traveled 379 feet, nearly doubling the stopping distance of the new-tire-equipped car. The eye-opener, says Rogers, was discovering that the sedan with 2/32nds tread was still going 44 mph when it reached the final stopping point of the 4/32nds-equipped vehicle.

The pickup truck test provided longer stopping distances but similar ratios, and the pickup with worn-out tires was still going 47 mph when it passed the stopping point of the 4/32nds-equipped truck. A video of the tests is available at tirerack.com by searching “panic stopping.”

Based on the tests, Tire Rack began recommending that drivers consider replacing their tires at 4/32nds tread. Rogers said customers may contend the company desires to drum up extra business, but viewing the tread depth video and determining if drivers will face wet driving conditions usually wins their confidence.

He explains that 2/32nds may be acceptable for an Arizona customer. However, if rain is common at the locale, then 4/32nds is appropriate for replacement. Similarly, snow, winter or all-season tires should be considered for replacement if only 6/32nds remain when heading into winter.

Consumer Push-Back
Rogers acknowledges that customers who replace tires at 4/32nds may not get the full benefit of a tread warranty or know if their tires are wearing out too fast, but tire wear is not linear.

“The first 32nd of treadwear may occur within the initial 1,500 miles of a new tire’s life, while its final 32nd could take 4,000 miles,” he says.

To determine the proper balance of the cost of new tires vs. performance, a consumer can compare the price of a new set of tires to their insurance deductible. Rogers points out that the deductible is likely higher than the cost of a set of new tires.

“As a tread drops below 4/32nds, the tire’s performance also drops and the potential for an accident increases,” he says. “A driver can pay the insurance deductible at the time of an accident, or invest in new tires and reduce the chance for an incident. Tires are inexpensive compared to the risk of a fender-bender’s damage and injuries.”

He adds that tire performance comes from its tread depth, compounds and pattern. Take away any of them and traction is sacrificed.

“Treadwear robs two of the components,” he says, “and at some point, the tires won’t meet a driver’s needs and expectations under certain conditions.”

Rogers is encouraged when customers embrace the concept that desired performance characteristics should dictate when to replace a vehicle’s tires.

“I’ve seen an owner of a 15-year-old Saturn with all-season tires recognize a loss of traction on a highway and invest in new tires before the old ones reached the 2/32nds wear bar.”

The Manufacturer Perspective
Rogers suggests that tire manufacturers haven’t embraced the quarter test because it contradicts the current 2/32nds warranty milestone. He’d like to see them adopt 3/32nds and 4/32nds pro-rated warranties to provide consumers with a more positive purchase experience.

Kurt Berger, Bridgestone Americas’ manager of consumer product engineering, says his company believes the federally-mandated 2/32nds minimum tread depth is prudent and provides a clear indication when to remove tires.

Berger agrees the decision to remove a tire prior to 2/32nds depends on several factors. He explains that wet and snow traction characteristics decline as a tire wears, but dry handling and rolling resistance can improve.

Bridgestone has developed technology for certain premium tires that incorporates a dual-layer compound to help maintain wet traction performance as the tread wears. For example, the Bridgestone Turanza exposes a second compound at approximately 50% tire wear to better maintain wet traction in the later stages of the tire’s life.

There are no separate worn-tire removal standards for snow tires, Berger said. The 2/32nd minimum wear bar still is applicable. However, most Bridgestone Blizzak snow tires also utilize a dual layer compound. The cap, or top layer, features the company’s multi-cell tread compound containing thousands of microscopic pores for enhanced grip on snow and ice.

These pores, Bridgestone contends, remove the thin layer of water bet­ween the tire and the snow or ice, increasing the tire’s ability to grip the surface. When the multi-cell compound is worn away at about 50% of the usable tread depth, a conventional winter tread compound is exposed and the tire can still be used in winter conditions with reduced traction levels as an all-season tire.

“It’s very important for customers to continually be aware of the condition of their tires,” says Berger. “Tire technicians can encourage regularly scheduled rotations and monthly inflation pressure checks to help prevent irregular or rapid wear. Our company wants to spread the message that regular tire rotation and proper inflation can help save money, enhance safety and optimize vehicle performance.”

He adds that creating a multi-level treadwear warranty to reflect variations in optimum tire replacement stages for dry, wet and snow performance is not being considered by Bridgestone at this time.

The penny and quarter tests add up to new opportunities for tire technicians to educate customers. Asking about their driving habits and anticipated weather and road conditions helps discover what drivers think they need, so recommendations and decisions can be made.

Driving habits and required vehicle performance should determine when to replace tires. Tire Rack’s Rogers explained that every tire dealer doesn’t have to produce a video to convince customers to switch out tires. There are simple, inexpensive ways to help them understand the importance of tire performance and how it’s impacted by tread depth.

Heck, the lesson might just cost 25 cents.

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