This paper analyzes four recent major studies carried out by MIT, a GM-led team, Directed Technologies, Inc., and A. D. Little, Inc. to assess advanced technology vehicles. These analyses appear to differ greatly concerning their perception of the energy benefits of advanced technology vehicles, leading to great uncertainties in estimating full-fuel-cycle (or “well-to-wheel”) greenhouse gas (GHG) emission reduction potentials and/or fuel feedstock requirements per mile of service. Advanced vehicles include, but are not limited to, advanced gasoline and diesel internal combustion engine (ICE) vehicles, hybrid electric vehicles (HEVs) with gasoline, diesel, and compressed natural gas (CNG) ICEs, and various kinds of fuel-cell based vehicles (FCVs), such as direct hydrogen FCVs and gasoline or methanol fuel-based FCVs. We focus on variations in estimates of vehicle gasoline-equivalent fuel energy use, glider and powertrain masses, and introduce powertrain effectiveness as a new surrogate measure for tank-to- wheel vehicle efficiency. We conclude that, while the degree of uncertainty across studies is considerable, it is not as great as a summary investigation and direct comparison implies. Our investigation suggests that there are logical and systematic reasons for variations among the studies. Further studies are required to improve both assessment and understanding of technical potentials of these advanced technologies, and to narrow the range of uncertainty currently present.