This paper summarizes octane number requirement trends of U.S. cars from 1971 through 1979, a period during which automakers strove to meet increasingly stringent fuel economy and exhaust emissions standards. It is based upon the annual nationwide surveys conducted by the Coordinating Research Council of current model domestic and imported light-duty vehicles sold in the United States. The maximum octane number requirement of the average 1971 model car was about 92 Research octane number (RON). After an initial drop of 3 units in the 1972 model year, average requirements increased throughout the remainder of the nine-year period to a level of 92 RON in 1979. Incidence of car owners reporting objectionable knock on the fuel they were using held at about 5% until 1975 when 10% objected. This number peaked at 20 percent in 1976 and then remained above 10 percent through 1979.