Before the Emergency Highway Energy Conservation Act each state could set their own speed limit within the range of 40 mph and 80 mph. However, around 1974, different countries from Europe and the United States supported Israel, which the Arab petroleum exporter countries did not like.
Therefore, the Arab countries stop sending oil to the United States, and this cause a rise in gas prices, which is why the president Nixon set the speed limit of 55 mph because Studies by the federal government showed that was the most efficient speed that consumed the less gas.