The research octane (RON) and motor octane (MON) numbers, carefully measured in standardized tests, are the principal parameters for characterizing autoignition of gasoline in engines. Increasingly, engines operate under different conditions of temperature, pressure, and mixture strength from those in these tests. As a result, RON and MON values become an incomplete guide to the onset of knock, and the octane index (OI), an octane number under operational conditions, is often measured. Values of the OI were measured with different fuels in a controlled autoignition single-cylinder engine, at different initial temperatures and pressures, at the instant of 10% heat release. Fundamental understanding of engine autoignition was sought by finding the OIs of different non-primary reference fuels (non-PRFs) by identifying the corresponding PRFs that give 10% heat release under identical conditions. The autoignition delay times of the PRFs were obtained from the shock tube data, for different temperatures and pressures, of the Aachen group. It was sufficiently accurate to equate the delay time of a non-PRF to that of the corresponding PRF under the same conditions for 10% heat release. The PRFs exhibited a higher value of the inverse pressure exponent for the delay time than the non-PRFs. Together with different temperature dependencies, these gave autoignition delay times of non-PRFs that could be higher than those of their associated RONs. This tendency increased with pressure and decreased with temperature and was most marked with olefenic and toluenic fuels. This could result in values of the OI that were higher than the RON of the fuel. This is important because, for a number of evolutionary reasons, engine pressures are tending to increase and temperatures to decrease.