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NEW YORK (CNN/Money) - Ford Motor Co. is warning customers that they should replace older tires, even if their treads aren't worn out.

On its Web site, Ford is now warning customers that tires should be replaced after six years, even if they aren't showing signs of wear.

"Tires degrade over time, even when they are not being used. It is recommended that tires generally be replaced after six years of normal service," the Web site says. "Heat caused by hot climates or frequent high loading conditions can accelerate the aging process. You should replace the spare tire when you replace the other road tires due to the aging of the spare tire."

The Detroit News reports that Ford is the first major auto manufacturer to issue such a warning, which has been advocated by some auto safety advocates in the past. It also said that the warning will appear in all owner's manuals for 2006 models and beyond.

The National Highway Traffic Safety Administration is not ready to issue any kind of consumer advisory, said agency spokesman Rae Tyson, but that does not prevent auto manufacturers or tire companies from issuing their own warnings if they have research suggesting a safety issue.

"Ford is to be commended if they want to step up and warn their customers," he told the Detroit News.

Ford, which was hit by billions in charges for recalling Firestone brand tires used on its vehicles in 2000 and 2001, said the move comes in response to a growing body of research that suggests tires deteriorate as they age and can experience tread separations and other failures, even if they look robust.

The Detroit News reports that foreign makers, including Volkswagen AG, BMW AG, Toyota Motor Corp. (Research) and DaimlerChrysler AG (Research), have alerted customers in Europe and the U.S. about aging tires. The European brands of General Motors Corp. (Research), Vauxhall and Opel also have carried similar warnings, but the company's core U.S. brands have not issued such a warning.

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