Streamline Vehicle Conversions Act

Streamline Vehicle Conversions Act - Amends the Clean Air Act to provide that a certificate of conformity issued by the Administrator of the Environmental Protection Agency (EPA) for one or more aftermarket conversion test groups with respect to an aftermarket conversion system (i.e., hardware installed on a light-duty or heavy-duty vehicle, light-duty truck, or heavy-duty engine that allows the vehicle or engine to operate on a fuel other than that which it was originally certified to use) shall not expire, shall continue to apply in subsequent calendar years, and shall not require recertification.

Requires the Administrator to: (1) establish criteria for use in identifying similar vehicle makes, models, original equipment manufacturers, emission standards, and different model years that may be used under a single test group; (2) accept such an established test group on an aftermarket conversion system manufacturer application for a certificate of conformity; and (3) allow the submission of previous vehicle emission test data for the certification of additional model year vehicles if the aftermarket conversion system manufacturer is able to demonstrate that neither the aftermarket conversion system nor the design and specifications of the applicable vehicle model are substantially different.

Directs the Administrator to promulgate regulations regarding on-board diagnostic systems for aftermarket conversion systems that: (1) ensure that such conversion systems that are equipped with on-board diagnostic systems are effective at monitoring critical emission components; (2) take into account the inability of an aftermarket conversion system manufacturer to access proprietary on-board diagnostic technology of an original equipment manufacturer; and (3) are sufficiently flexible to encourage the increased use of alternative fueled vehicles.

Sets forth provisions regarding the conversion of a vehicle outside of its useful life cycle.