Many years ago I saved an article from Consumer Reports where they were addressing the demise of unleaded premium while leaded regular still existed. Suprisingly, lab studies of 50% 89.1 leaded regular mixed with 50% 91.6 unleaded premium offered an octane number of 91.2 which is much closer to the premium number than a proportional calculation. As they put it, "...the first bit of added lead has a much greater impact of raising octane level of gasoline than the rest...."
