How safe are worn tires? Even tires with half their tread intact may be riskier than you think.(Auto Parts)

Tires can lose their footing long before they're worn out. Our tests show that tread can give up a significant amount of grip when it's still at the halfway point.

That's particularly worrisome when you realize how many worn tires are on the roads. A recent study by the National Highway Traffic Safety Administration found that nearly 50 percent of the 11,500 cars, pickup trucks, vans, and sport-utility vehicles the agency checked had at least one tire with half-worn tread. Another 10 percent had at least one bald tire.

Worn tires--especially bald ones--can be deadly on wet roads, where the grooves aren't deep enough to channel water out from beneath the tread. The result is hydroplaning, where the tread skims the water's surface and the vehicle no longer responds to the steering wheel. Wet-weather braking and snow traction also decrease as tires wear.

Tires are considered bald when one or more of their grooves reaches 232 of an inch deep, compared with about 1032 of an inch for new tires (tread wear is usually measured in 132-inch increments). Manufacturers have made bald tires easier to spot by placing a series of molded horizontal bars at the base of the grooves. The bars become flush with surrounding tread when wear reduces a groove's depth to 232 of an inch. That's also the point where tires will flunk a state safety inspection--and where tread must be worn for you to collect on a tire's tread-wear warranty.

Unfortunately, 232 of an inch may be too late if you drive in rain or snow. Based on our tests of new and half-tread-depth tires, you may want to consider replacing the ones on your car or truck closer to the 532-inch groove depth that marks the half-tread point on many tires.

FOUL-WEATHER FACE-OFF

We tested all-season passenger-vehicle tires, which account for more than 90 percent of all replacement tires sold. Our tests included three high-scoring models from our November 2001 report: the premium Kelly Navigator Platinum TE, the midline Firestone FT70c, and the economy BFGoodrich Momenta S/E. All three cost between $50 and $60 each in the P195/ 70R14 size we tested, and represent the type of tires many people buy.