Improving the efficiency of the vehicle fleet and diversifying available vehicle technologies are critical strategies for strengthening transportation system resilience and reducing dependence on imported fuels. One type of public policy to achieve these goals is performance-based regulations, which act upstream in the supply chain by requiring vehicle manufacturers to meet a specified standard across a fleet. A key feature of modern vehicle regulations is the inclusion of flexible design mechanisms that allow manufacturers to choose among multiple compliance pathways. These mechanisms—such as credit trading and banking—are intended to reduce compliance costs and improve policy acceptability. However, despite their widespread use, there is limited systematic understanding of how these design features are defined, their rationale for implementation, and how they influence regulatory outcomes. This project will develop a comprehensive framework for understanding and designing flexible vehicle efficiency regulations. The work will identify flexible design mechanisms embedded in global vehicle efficiency regulations, analyze how different policy design features influence compliance costs and cost distribution across manufacturers, and provide policy-relevant guidance for designing efficient, durable, and adaptable vehicle regulations.