Each of forty-eight vehicles was driven in urban and suburban modes of operation for at least 12,000 miles. Three groups of 16 cars each were operated on the same base fuel, with two of the fuels modified with CRC(Coordinating Research Council, Inc.)-provided additives. Exhaust emission, PCV valve flow, and Idle RPM (for a total of 19 dependent variables) were measured when the vehicles started the test and at each subsequent 4,000-mile interval through 12,000 miles. End-of-test measurements of the same 19 dependent variables were made on a subset of vehicles driven more than 12,000-miles. Analysis of the data shows that vehicles operated on fuels containing the gasoline additives had lower carbon monoxide emissions than those operated on the nonadditive fuel. The lower carbon monoxide emissions were accompanied by increases in carbon dioxide.