A program was undertaken to determine which retrofit methods are currently feasible for used cars, considering emission reduction effectiveness, cost, effects on vehicle performance, facilities, and labor skill required for retrofit installation, maintenance, and inspection.An extensive search was made for all sources of information on retrofit methods, developers, and producers. Eleven representative retrofit devices were actually tested in this project. A fleet of 20 used vehicles without factory installed exhaust control systems was used for testing the performance of the retrofit devices.The vehicles were tested first without a retrofit device to obtain a reference baseline, and then with retrofit device installed. Emission tests were conducted by the 1972 CVS Federal Test Procedure. Fuel consumption was also measured during the CVS test. Drivability tests were conducted by an Automobile Manufacturers Association procedure. Durability tests were conducted on four types of retrofit devices over a representative driving pattern for 25,000 miles.To evaluate and compare the different retrofit systems, an objective evaluation methodology was developed to provide quantitative figures of merit or indexes in terms of feasibility criteria, performance, and cost-effectiveness.A summary of the results of the retrofit tests, analyses, and evaluations are presented.