Higher speed limits on rural interstate highways are having no discernible effect on traffic fatalities, probably due to stricter enforcement, Transportation Secretary Jim Burnley says.

"A 55 mph limit that is not enforced can be more of a problem than a 65 mph limit that is enforced," Burnley said this week after a speech to the Society of Automotive Engineers.A law enacted last year over President Reagan's veto allows states to raise the speed limit to 65 mph on rural stretches of interstate highways, 10 mph faster than the national 55 mph speed limit enacted in 1974 as an energy conservation measure.

Forty states have adopted the higher speed limit.

Burnley declined to provide specific figures to support his conclusion, saying they will be provided to Congress in a report being prepared by the Transportation Department.

"We're just not finding any distinction" between fatalities on the 65 mph rural interstate highways and those where the speed limit remains at 55 mph, Burnley said.

The fatality statistics "are all over the board," with deaths rising in some states and falling in others on 65 mph stretches of highway, said Transportation Department spokesman Ron DeFore.

The only official statistic on the issue has been provided by Diane Steed, administrator of the National Highway Traffic Safety Administration, in congressional testimony. She said that in the months after the federal law took effect, the agency found increases between 18 percent and 23 percent on rural interstates in states that raised the limit.

DeFore, however, said it is difficult to assess the effect of the higher speed limit because of possible variations in other safety factors, such as enforcement.

In his speech, Burnley said Americans "are becoming more safety conscious," citing a 14 percent decline in the number of drunken drivers involved in fatal auto accidents between 1982 and 1986.

But the National Safety Council told Congress it believes the higher speed limit is causing more deaths.