The replacement of conventional materials with lightweight materials can improve vehicle fuel economy and associated emissions, but the choice of materials is dependent on complex design requirements. Carbon fiber reinforced polymer (CFRP) composites have high stiffness and tensile strength with relatively low mass, and are increasingly being deployed in light-duty vehicles (LDVs). CFRP composite life cycle energy advantages are a balance between highly energy-intensive production processes and the energy savings and greenhouse gas emissions reductions that mainly occur in the use phase for applications such as transportation. In the production phase, the manufacturing energy intensity of CFRP composites is greater than that of conventional metals. A review of commercially available manufacturing methods estimates the primary energy intensity of CFRP composites with 50% fiber volume fraction to be roughly 800 MJ/kg, whereas that for conventional steel is only 50 MJ/kg [1]. Conventional steel is produced by processes that are well established and have undergone over 150 years of optimization and energy intensity improvements, while CFRP composites are currently produced by relatively new processes that have promising opportunities for optimization and energy intensity improvements. This analysis explores the substitution of a conventional steel LDV door with a carbon fiber reinforced polymer having a 50% fiber volume fraction (50%CFRP). The energy intensity of the 50%CFRP composites is assessed for three scenarios corresponding to the current typical manufacturing techniques, the most energy efficient commercially available techniques, and the practical minimum energy intensity based on applied research technologies with commercial potential identified in an initial investigation. The effect of these variations in manufacturing energy intensity on the life cycle analysis is shown using the LIGHTEnUP Tool (Lifecycle Industry GHgas, Technology and Energy through the Use Phase) developed by the U.S. Department of Energy (DOE) and the Lawrence Berkeley National Laboratory (LBNL). The break-even manufacturing energy intensity threshold for the 50%CFRP composite LDV door to provide a net reduction in life cycle energy and associated carbon dioxide emissions is computed. The paper concludes with a discussion of the practical drivers affecting the design choices, manufacture and use of CFRP composites in LDVs.