 The catalytic emission control system was more efficient at reducing emissions than the previous non-catalytic system, and allowed engines to be retuned for improved drivability and fuel economy.The catalytic converter and GM High Energy electronic ignition were advertised among the components of "Chevrolet's new Efficiency System" which was promoted to offer other benefits to owners of 1975 models versus comparable 1974 models that included extended maintenance intervals from 6,000 to for oil/filter changes and spark plugs that lasted up to compared to on 1974 models.The Z28 option was discontinued for 1975 despite an increase in sales to over 13,000 units in 1974.