Test Your Tires: Use a Quarter

New study shows penny test is outdated; tire must have an eighth inch of tread.

ByABC News via logo
October 3, 2007, 9:40 AM

Oct. 3 , 2007 — -- Forty-three states require motorists to get new tires when the tread wears down to a 16th of an inch, but new studies show that may be too late a timely warning with the fall rainy season coming.

When driving, only a tiny portion of each tire is in contact with the road, and that's why experts say it's so important that your tires be in good shape.

A company called Tire Rack recently took three sets of the same tires each with different amounts of tread and conducted road tests in wet conditions. Using the same driver at the same speed, the company found the tire with only a 16th of an inch of tread took far longer to come to a stop.

For years, motorists have been told the "penny test" is an accurate indicator of whether or not you need new tires. The test is conducted by sticking a penny head down in a tire tread; if you see all of Lincoln's head, then you should change the tires.

Tire Rack, which studies and sells tires, says that the penny test is outdated and that drivers should use a quarter instead. The quarter gives a measurement of an eighth of an inch instead of a 16th.

"The difference between the quarter-tested tires and the penny-tested tires can be the difference between stopping on a wet expressway or crashing into the car ahead of you," said John Rastetter, Tire Rack's director of tire information services.

In Tire Rack's test, the car with brand-new tires stopped the fastest. The one with an eighth of an inch of tread stopped in 300 feet. At the 300-foot mark, the car with just a 16th of an inch of tread continued to barrel by at 44 mph.

"The video certainly was impressive and we're not at all surprised by what we saw in it. It's that extra tread depth that gives you the margin of safety when the weather turns bad," said John Nielson, director of automotive repair for AAA.

Adequate tire tread is especially important in rainy conditions because it gives the water a way to escape so your car doesn't hydroplane. In 2005, about 3,000 people died in rain-related accidents, according to the National Highway Traffic Safety Administration.