This paper reviews 20 years of data dealing with customer reaction to gasoline octane quality. Four technical studies and one marketing survey are covered. An analysis of the data shows that customer reaction to engine knock has not changed during this period. Agreement among three of the technical studies was good. ΔRON, the difference in Research Octane Number requirements between trained observers and customers, varied from 3.0 RON at the 50th percentile satisfaction level to 1.7 RON at the 90th percentile satisfaction level. The marketing survey shows that these values of ΔRON are a good measure of the number of customers in the marketplace who consider knock to be a problem. The fourth technical study measured ΔRON to be at least twice as large as the values in the other studies. This discrepancy is traced to the design of the questionnaire used to evaluate customer reaction to knock.