The National Highway Traffic Safety Administration yesterday granted U.S. auto makers a two-year reduction in federal fuel economy standards for pickup and related trucks often used in the construction and retail industries.

The trucks also are gaining popularity as personal-use vehicles.

NHTSA'a waiver of the corporate average fuel economy (CAFE) standards will affect light trucks manufactured for the 1985 and 1986 model years.

CAFE standards set the annual minimum average fuel-economy rating that vehicle manufacturers must achieve in their new-car and light-truck fleets.

Under the revised rules -- requested by Ford Motor Co. and supported by General Motors Corp., the nation's two largest auto makers -- the CAFE standard for the 1985 model year will be 19.5 miles per gallon, down from an originally scheduled 21 mpg. The standard for 1986 will be 20 mpg, down from 21.5 mpg.

Conservationists and officials of Chrysler Corp. long had criticized efforts to reduce CAFE standards as being foolhardy and contrary to the nation's supposed drive toward energy efficiency.

But the Department of Transportation, which has jurisdiction over NHTSA, said yesterday that the changes were needed to reflect the realities of what is happening in the marketplace.

"Due to a continuing shift in consumer demand for light trucks, the agency anticipated that downward revisions of the standards would be necessary," DOT officials said.

"The demand shifts, which are due primarily to the recent trend of stable and diminishing gasoline prices, are manifested in higher levels of sales of larger light trucks and larger-displacement engines than were previously anticipated by either the manufacturers or the agency," the DOT statement said.