Penny to Quarter: The New Tire Test
Can Honest Abe no longer be trusted for the tire test?
Blame it on inflation. For years the de facto standard for determining how much rubber remained on your tires was to use a penny. Depending on how much of Lincoln’s noggin was visible when the coin was placed in the tire’s treads, you knew if it was time for a new set of shoes for your ride. Now nationwide retailer The Tire Rack says the price of the test has gone up 2500 percent. The Tire Rack claims the so-called Penny Test should be replaced by the Quarter Test if you want to be certain your tires are safe for wet-weather driving.
Devaluing the Penny
Developed decades ago as a quick way to check your tires for tread wear, the Penny Test consists of placing the coin upside down in a tire’s tread grooves. If the top of Lincoln's hair is visible, it indicates that the tread has worn to less than 2/32 of an inch (1.6mm). In some states it’s illegal to drive with only that amount of tread wear remaining on a tire. Even if it’s not the law where you live, it can be downright dangerous to rely on so little rubber for traction in emergency situations.
After testing tires that pass the Penny Test on a water-slicked track and comparing their grip to tires with a tread depth of 4/32 of an inch — or the amount of tread that’s left when you place a quarter in a tire’s grooves and can still see Washington’s scalp — The Tire Rack says that small amount of extra rubber makes a big difference in stopping distances. At highway speeds with 2/32 of an inch of tread depth remaining, resistance to hydroplaning (when a tire “floats” over water on the road instead of gripping the pavement) is significantly reduced and stopping distances dramatically increase.
“Our recommendation is intended to get people to recognize that they’re going to be sacrificing wet stopping ability by letting their tires get to a 2/32 of an inch tread depth," says John Rastetter, director of tire information at The Tire Rack. “And we want them to consider that rather than driving on their tires longer.”
The Tire Rack came to its “two-bit” conclusion after conducting an independent test that compared the stopping distances of tires with 2/32 of an inch of remaining tread depth to tires with 4/32 of an inch of tread depth and to new tires. The tests were conducted at a 70-degree ambient temperature on a 600-foot long asphalt braking lane using a 2006 BMW 325i and a 2006 Ford F-150 SuperCab 4x2 pickup driving at 70 mph. Both vehicles were equipped with four-wheel disc brakes and anti-lock brake systems, and with tires that had been used as original equipment on equivalent models of the cars.
One set of test tires had full tread depth, while the other two sets were shaved to 4/32 and 2/32 of an inch of remaining tread and baked in an industrial oven at temperatures that replicate aging the rubber over several years of driving. All tire sets were broken in for approximately 100 highway miles. The asphalt test track was wetted by a watering system that maintained 0.05 to 0.06 an inch of water depth above the peaks of the asphalt's aggregate. To bring another coin measurement into mix, this means you could lay a dime on the road and the water would flow around but not over it. The testers accelerated to just above 70 mph on dry road and then drove across about 75 feet of wet pavement before slamming on the brakes and recording a panic-stopping distance.