Some concern has been voiced that a large portion of 1975 model year automobiles will have engine knock problems when operating with unleaded gasoline of 91 RON/83 MON. Unleaded fuel of 91 RON/83 MON is the only unleaded gasoline that will be available at most of the retail pump outlets. This will occur because most of the gasoline companies will limit their production to satisfying only the minimum requirements of the current EPA regulations. These regulations require availability of unleaded gasoline of 'not less than 91 Research Octane Number'. The concerned groups indicate that the percentage of cars satisfied with this unleaded gasoline will be much lower than the percentage of past years. These concerned groups point out that too many cars will have to resort to spark retard for elimination of knock, and that this will result in a substantial loss in fuel economy. The purpose of this report is to document and discuss, concisely, that information which is relevant to this potential problem concerning engine knock.