WASHINGTON — American manufacturers--written off by many commentators in the 1970s and '80s as dinosaurs doomed to succumb to Japanese and other foreign rivals--have staged a remarkable comeback, reviving American competitiveness in many industries.

Xerox Corp. has halved the cost of producing a copier, and has steadily increased its share of the U.S. market since the mid-1980s. General Electric Co.'s exports have grown more than 20%, to $6 billion, in the past two years. Cummins Engine Co., the largest American manufacturer of heavy-duty diesel truck engines, has doubled its output per worker since 1985 and cut prices of its engines by nearly a third.

Henry B. Schacht, president of Cummins, predicted in a recent interview that after the current recession ends, "the U.S. will be ferociously competitive in manufacturing. . . . It's a great place to be in business, a marvelously competitive base."

Such euphoria is not universal, and U.S. firms still face daunting competition from Europe, Japan and new economic powerhouses such as South Korea and Taiwan. Recent studies of technological competition predict that Japanese firms will continue to chip away at the American lead in many key markets.

The determination of firms such as Cummins to stay competitive internationally has cost a lot at the bottom line; profits have been down in recent years, and nonexistent since mid-1990, as a long-running slowdown for truck makers tooks its toll.

But a five-year growth of American exports is strong evidence of restored competitiveness, according to many economists and business leaders. In the latest figures, released last week, U.S. sales overseas rose in March to their third-highest monthly level ever.

Part of the surge in exports can be explained by a dramatic lowering of the dollar's value, compared to currencies in Japan and Europe. Engineered in 1985 by the Reagan Administration amid a crisis for American manufacturers, the change provided U.S. producers the opportunity to cut prices sharply on goods sold abroad.

Exchange rate adjustments alone do not explain improving performance by U.S. firms, however. According to foreign and American business executives and experts, many U.S. companies have radically improved the quality of their products, cut costs and improved efficiency, and generally shown a willingness to learn lessons taught to them painfully by Japanese, German and other competitors.