A federal appeals court yesterday upheld the Reagan administration's relaxation of automobile bumper standards, but auto safety and insurance groups said they will continue to press for stronger standards.

The U.S. Court of Appeals, in a 2-to-1 opinion, ruled that the National Highway Traffic Safety Administration did not act wrongly in May 1982 when it required auto bumpers to withstand impacts of 2.5 miles per hour, replacing the previous 5 mph standard.

Clarence Ditlow, director of the Center for Auto Safety, called the appeals court ruling "a major consumer defeat," and said the center may ask the full court to rehear the case.

An attorney for State Farm Insurance Co. called upon Transportation Secretary Elizabeth Hanford Dole to consider issuing a new rule based on "current evidence that the 2.5-mph bumpers are a consumer disaster."

However, the Motor Vehicle Manufacturers Association "is very happy" with the decision, said spokesman Al Rothenberg, praising the court "for deferring to NHTSA's expertise."

NHTSA had argued that the lower standard was more cost-effective because auto makers could install bumpers that weigh less. The lighter bumpers would increase fuel efficiency and could ease pressure on auto prices because they would cost less to manufacture, the agency reasoned.

"In setting the standards in 1982, NHTSA sought an adequate level of protection which provided the maximum feasible reduction of costs to the consumer," Diane K. Steed, the agency's administrator, said in a statement issued yesterday.

But safety and insurance groups challenged the rule, disputing the numbers used by the government to determine cost effectiveness and contending that NHTSA "did not consider safety," Ditlow said.

The Motor Vehicle Manufacturers Association contends that "in some cases," the heavier bumpers "do more damage than good," Rothenberg said.

Ditlow said NHTSA underestimated the cost to consumers in time and money of having to repair damage to bumpers and other car parts.

The promised benefits of lighter bumpers also failed to materialize, he said, contending that the switch to lighter bumpers in the middle of the 1983 model year yielded no difference in sticker price or fuel economy.

Ford Motor Co. and Toyota Motor Corp. have voluntarily retained the 5 mph bumper standard, and General Motors Corp. has said that its 1986-model-year cars will have the heavier bumpers.

Front and rear bumpers act more to prevent damage to the car than to the passenger. Ditlow said about half of all 1985 cars have bumpers that prevent any damage to the car in a 5-mph impact. Many of those heavier bumpers, however, must be replaced after such an accident because they are seriously damaged in such crashes, he said, estimating average replacement costs at $150 to $200