Emissions and fuel economy have been measured for two years on a group of 56 catalyst vehicles in consumer use and maintenance. Data presented here include idle CO; mass emissions of HC, CO, NOx, sulfate, and SO2; and fuel economy. Test cycles used were the 1975 FTP, one hour 50 mph cruise, CFDS and HFET. Fuel economy data are also presented for the same vehicles for over-the-road driving from on-board totalizing fuel and engine hour meters on each car. Sulfur dioxide emissions characteristic of two identifiable modes of carburetor tune were observed upon deceleration to idle and are discussed. Catalyst activity was measured at idle, 30 and 50 mph by exhaust sampling before and after the catalyst. Although 70% of the catalysts were established to be active at 30 and 50 mph, catalyst activity at idle was observed to be much lower when CO was > 1%.