Despite the shroud that often envelops the executive suites at GM's world headquarters, every now and then a solid idea does bet loose and float down to the troops. This one's a "good-un." GM says it will become the first automaker to cover tires under new car and truck warranties. Tires are in no-man's-warranty land, leaving customers to seek relief not from car or tiremakers, but from tire dealers who didn't sell them the rubber in the first place. Under the new policy (Saturn is an exception), GM's dealers will handle customer complaints about tires and get tire manufacturers to pay warranty costs. WAW will go out on a limb as thin as an oak tree's and predict that this customer service will become an industry norm by the `98 model year. GM's competitors seem certain to follow suit.