What happened to gasoline in the mid 1970s?

I remember that the whole effort was to reduce the emission of oxides of nitrogen (NOx) which was the cause of photochemical smog in major cities. It was found that higher cylinder temperatures resulted in higher amounts of NOx and, by reducing the compression ratio of the cylinder, the reaction temperature was lowered enough to reduce the amount of NOx. The catalytic converters could then be more efficient in breaking up the NOx products. The lower compression ratios allowed for more “regular” gas to be used instead of the higher octane rating gas.