Under pressure from the U.S. auto industry, the Department of Transportation yesterday relaxed the fuel economy standards it will require for pick-ups and vans starting in 1980.

DOT will require light trucks to average at least 16 miles per gallon in 1980 and 18 miles per gallon in 1981, Secretary of Transportation Brock Adams announced.

Earlier DOT had proposed a standard of 19.2 miles per gallon starting in 1980. Auto industry sources estimate that 1979 trucks will average 14.6 miles per gallon. Current models get slightly worse mileage.

The big three car makers, who had denounced DOT's original truck mileage goals as unrealistic, if not impossible to obtain, generally supported the new standards.

Chrysler Corp., which warned it would have to close a Detroit truck factory if the 19 mpg standard were imposed, said the 16 mpg figure was "within the ballpark". Ford called the new standards "more realistic."

But Clarenec Ditlow, director of Ralph Nader's Center for Auto Safety. said easing of the standards show that "DOT caved in to the auto industry."

Acknowledging that DOT had been expected to ease the standards somewhat, Ditlow predicted the fuel economy regulatons "are going to cause a severe cramp in the Carter administration's fuel conservation plans."

Adams said the the new standards will save eight billion gallons of fuel over the life of the vehicles. The stiffer standards which DOT proposed in December, would have saved 12 billion gallons of gasoline.

The new standards apply to pickups the vans with a gross vehicle weight rating (loaded) of up to 8.500 pounds. Last year DOT set a 1979 mileage minimum of 17.2 miles per gallon, but limited it to vehicles of up to 6,000 pounds. The 16 miles per gallon minimum for 1980 is the average gas mileage required for all two-wheel drive, light trucks made by a manufacturer. Four-whell drive vehicles will have to get an average of 14 miles per gallon by 1980 and 15.5 miles per gallon by 1981.

DOT said the mileage requirements for 1981 light trucks would save the owner about $600 in fuel cost over the vehicle's life, compared with present trucks. It will cost the truck makers about $60 per vehicle to meet the standards, producing a net savings of $450, the agency said.

The Council on Wage and Price Stability had called the original proposal inflationary, saying it would add $160 to the price of every pickup or van.

Even at the cost, the auto makers said, they would be forced to stop making some trucks because the vehicles could never meet the standards.

Chrysler, whose Dodge trucks have the worst mileage ratings of the Big Three, said it would have to close a Detroit plant because it could not meet the 19.2 mpg standard. The National Association for the Advnacement of Colored People backed Chrysler's request for easing the standards, saying hundreds of black workers would lose their jobs if Chrysler closed the plant.

Chrysler said it would issue an official announcement on the new standards today, but a spokesman called the new standards "quite a roll back." He said the 16 mpg standard is "certainly within the (acceptable) range."

But the Chrysler official said the 18 mpg standards for 1981 pickups and vans is "more than the company can do right now." Chrysler has estimated its 1980 trucks will get 13.9 miles per gallon and the 1981 models, 14.78 mpg.

Ford Vice President Herbert L. Misch said DOT "reacted thoughtfully to our estimates of what we can accomplish." Ford expects to introduce a new line of lighter weight trucks in 1980.

The new fuel economy requirements do not apply to "captive imports" which are built overseas and sold here, such as the Chevrolet Luv and Ford Escort pickups. Those trucks, which generally get better gas mileage from their four-cylinder engines, cannot be counted in meeting the 16 mpg 1980 average.