Pointing to mounting consumer debt and the prospect of higher interest rates, Tom Webb, economist for the National Automobile Dealers Association, predicts that light-vehicle sales in the United States will fall next year to about 14.5 million from somewhere above 15 million this year. Webb even warns that the rosy outlook for this year could yield to stormy skies if the UAW and General Motors Corp. fail to find common ground. The big bugaboo: 1.1 million families will file for bankruptcy this year, despite lower unemployment and generally strong economic growth. "In 1996 households were drunk with debt, but they still staggered back to the bar for another drink," Webb tells a conference in Washington last month. "In 1997, I think the bartender will refuse to serve."