Vehicle purchase price incentives in the form of fees and rebates (" feebates") linked to vehicle efficiency are a potential mechanism for reducing US transportation energy consumption and greenhouse gas emissions. General principles for formulating feebates are described, and issues to be addressed in developing a workable feebate program are identified, particularly treatment of light trucks and differential impacts on manufacturers. Size-adjusted feebates are introduced as a way to mitigate the intermanufacturer equity concerns that arise with a system based only on fuel consumption. The impacts of various feebate formulations are examined by a nameplate-level static analysis of the 1990 fleet. By separating cars from light trucks and choosing an appropriate size-adjusted approach, it is possible to develop a feebate system that does not disadvantage US automobile makers. The data base analyzed also reveals a significant variation among different configurations of a given nameplate. Although further analysis is needed, the size-based feebate concepts presented here may serve as a foundation for designing vehicle price incentives to reduce energy use and carbon dioxide emissions.