With the vast majority of vehicles in the United States designed to operate on gasoline or diesel fuel, there has been a long and growing interest by the public in clean alternative fuel conversion systems. These systems allow gasoline or diesel vehicles to operate on alternative fuels such as natural gas, propane, alcohol, or electricity. Use of alternative fuels opens new fuel supply choices and can help consumers address concerns about fuel costs, energy security, and emissions. EPA supports such innovation and encourages the development of clean aftermarket technologies that enable broader transportation fuel choices. At the same time EPA is responsible for ensuring that all vehicles and engines sold in the United States, including clean alternative fuel conversions, meet emission standards. EPA is adopting a new approach that simplifies and streamlines the process by which manufacturers of clean alternative fuel conversion systems may demonstrate compliance with these vehicle and engine emissions requirements. The new options will reduce some economic and procedural impediments to clean alternative fuel conversions while maintaining environmental safeguards to ensure that acceptable emission levels from converted vehicles and engines are sustained. The final rule covers alternative fuel conversion of light-duty vehicles and heavy-duty highway vehicles and engines.