The U.S. Environmental Protection Agency, U.S. Department of Transportation's National Highway and Traffic Safety Administration, and the California Air Resources Board have recently released proposed new regulations for greenhouse gas emissions and fuel economy for light-duty vehicles and trucks in model years 2017-2025. These proposed regulations intend to significantly reduce greenhouse gas emissions and increase fleet fuel economy from current levels. At the fleet level, these rules the proposed regulations represent a 50% reduction in greenhouse gas emissions by new vehicles in 2025 compared to current fleet levels. At the same time, global growth, especially in developing economies, should continue to drive demand for crude oil and may lead to further fuel price increases. Both of these trends will therefore require light duty vehicles (LDV) to significantly improve their greenhouse gas emissions over the next 5-15 years to meet regulatory requirements and customer demand. In this paper, technology pathways leading to improved light-duty vehicle fuel consumption are described as well as the complex systems methodology used to assess future technology combinations.The complex systems methodology described here was initially developed by Ricardo for use by the U.S. EPA to analyze the effectiveness of future light-duty vehicle and truck technologies. The complex systems methodology first defines the major vehicle systems, such as powertrain configuration, engine, transmission, and other vehicle parameters, such as vehicle road load, that constitute the design space of the study. The design space is then efficiently sampled using a Design of Experiments (DoE) methodology, and the results are synthesized into a parametric model that can be used to efficiently assess benefits of single configurations or groups of configurations within the vehicle design space. Estimates of future fuel economy results for various vehicle configurations, and the influence of technology package development are also presented; however. the emphasis of this paper is on the tool and the methodology. For a description of the actual application of this tool in support of the model years 2017-2025 light duty greenhouse gas and fuel economy proposal, the reader is referred to the Proposed Rule .