Transitioning to a cleaner fleet of advanced vehicles powered by electricity, hydrogen, and advanced biofuels or petroleum products can yield a significant reduction in greenhouse gas emissions and petroleum consumption. A meaningful assessment of the comparative merits of these alternate fuel pathways requires a solid understanding of their technological potential to reduce emissions. Available studies evaluating full lifecycle emissions rely on various assumptions of that potential and yield a wide range of results. This brief summarizes and synthesizes the results of several recent studies and presents the full range of greenhouse gas emission estimates for each type of advanced vehicle and fuel. It also explains the reasons these estimates vary so widely and identifies opportunities for future analyses that use a consistent set of scenarios with transparent assumptions in order to compare the greenhouse gas impacts of fuel and vehicle pathways.