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When to replace your vehicle’s tires

Consumer Reports engineers have looked at the issue of when drivers should consider replacing their vehicle’s tires. Tires are legally “worn out” when their tread depth reaches one-sixteenth of an inch. “Tread wear indicator bars” moulded into the tire visually show consumers when their tires have worn down to that depth.

But Consumer Reports’ tests have shown that, for safety sake, consumers should replace their tires sooner—when they have worn to one-eighth of an inch.

Performance in wet-braking, wet cornering, and hydroplaning resistance tests all dropped off significantly for very worn tires. Replacing tires before they have worn down to the tread wear indicator bars would provide an added safety margin for drivers.

To get a handle on how much tread depth makes a difference, Consumer Reports tested two models of V-rated performance all-season tires—a kind widely available on new cars. CR subjected sets with full tread depth, one-eighth-inch tread depth and one-sixteenth-inch tread depth to a battery of performance tests. (Tires were shaved by a machine to simulate a worn condition so the effects of tire aging could not be taken into account.)

Handling, cornering and dry braking actually improves a bit as tires begin to wear. But hydroplaning resistance and traction on snow deteriorates as tread depth is lost. By the time only one-sixteenth of an inch of tread was left, performance in wet-pavement cornering and braking tests had also deteriorated.

It’s also possible to use a coin to measure tread depth. For those in the United States, if you don't have a tread depth gauge, you could hold a U.S. penny upside down in the tread. Under the old standard, if the top of Lincoln’s head on the penny was visible, the tires were worn to one-sixteenth of an inch and you needed new ones. With newer Canadian pennies, the same applies if you can see the top of the word "Elizabeth" beside the Queen's head, or on a British penny if you can see the Queen's head.

Consumer Reports engineers are now recommending that you measure for one-eighth of an inch of tread depth, using a quarter rather than a penny. The distance from the coin’s rim to George Washington’s hairline is about one-eighth inch. If you see George’s head, consider replacing your tires. On a Canadian or British penny, you would see the top of the Queen's forehead.

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More importantly, replace older tires

In addition, you should replace any tires that are more than six years old. As this ABC News report on older tires illustrates, old tires can deteriorate quickly and cause traumatic accidents. You should check the date code (a four-digit number after the DOT data on tires made since 2000, or a three digit code on tires made before then) to ensure any new tires you buy are have not been sitting on a dealer's shelf for years.

A three-digit date code of 447 would mean a tire was made in the 44th week of 1997. The four-digit code 1504 would show a tire was made in the 15th week of 2004. According to most tire manufacturers, no tire should be used on your car if it is more than six years old.

Posted Oct. 7, 2007. Updated Nov. 20, 2009.©

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