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PRESENT methods of correlating road knock behavior of fuels with laboratory methods are unsatisfactory and should be discarded in favor of a method that makes a new approach to the problem, the authors contend. Reviewing the causes of this situation, the paper offers evidence to show that the use of mathematical averages applied to the study of the car detonation problem has been very misleading and that the so-called average fuel octane-number requirement of existing cars and its corollary, the octane requirement of the average car, are values having little practical significance. A procedure is recommended by the authors as one approach to the problem of setting up a simple, practical way of evaluating fuels. It consists in selecting a number of privately owned cars and testing both reference-fuel blends and branded fuels in these cars without making any changes whatsoever in the engine adjustments. The results of two such surveys are reported.