The 65-m.p.h. speed limit caused 550 deaths last year, increasing the death toll on rural interstate highways twice as much as shown in earlier studies, an automobile safety group said in Washington. A federal highway official also said that 1988 fatality data indicates higher speeds on 65 m.p.h. highways may be causing more deaths, but a spokesman for a motorists’ group countered that increased speed limits do not kill. Brian O’Neill, president of the Insurance Institute for Highway Safety, financed by insurance companies, told a House panel that actual deaths on rural interstates increased 34% in 38 states which raised the speed limit to 65 m.p.h. in 1987. But Will Fox, spokesman for Citizens for Rational Traffic Laws of Dane, Wis., told reporters after the hearing that higher speeds do not kill more people. He said the fatality statistics are misleading because they do not factor in the number of miles traveled.