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Annually, the Coordinating Research Council (CRC) sponsors a nationwide survey of octane number requirements for current model passenger cars. In this review, based on surveys from 1965 through 1974, trends in octane number requirements reflect changes in engine design and exhaust systems to meet the legislated limits on emissions. The average requirement dropped approximately 5.5 Research octane numbers over the ten-year period with the most dramatic drop of approximately 3.0 octane numbers in 1971 when car manufacturers recommended use of 91 octane gasoline. The trends toward lower average requirements since 1971 have been determined with both leaded and unleaded fuels. Incidence of surface ignition knock and rumble was about one per cent in 1965 and tended to disappear after 1971. The average spread of octane number requirements between 10 and 90 per cent of cars satisfied increased about 2.5 numbers over the period.