Vehicle lightweighting has been a focus of the automotive industry, as car manufacturers seek to comply with the corporate average fuel economy (CAFE) and the greenhouse gas (GHG) standards for MY 2017-2025 vehicles. However, when developing a lightweight vehicle design, the automotive industry typically targets maximum vehicle weight reduction at minimal capital cost. In this paper we consider the environmental impacts incurred by some of the lightweighting technology options. The materials generally used for vehicle lightweighting include high-strength steel (HSS), aluminum, magnesium and carbon fiber reinforced plastic (CFRP). Except for HSS, the production of these light materials is more GHG-intensive (on a kg-to-kg basis) compared with the conventional automotive materials they substitute. Lightweighting with these materials, therefore, may partially offset the GHG emission reductions achieved through improved fuel economy. To retrospectively investigate the impact of this burden shifting from vehicle operation to vehicle production due to material substitution, historical weights, material compositions, fuel economies , and other characteristics of the U.S. light-duty fleet for 1995-2014 are presented and analyzed in this study. The GHG emissions from vehicle production, vehicle operation, and the entire life-cycle of the vehicle, are then estimated with Argonne’s Greenhouse gases, Regulated Emissions, and Energy use in Transportation (GREET®) model. A comparison of the GHG emissions of a conventional MY 2014 and lightweighted MY 2015 Ford F150 is also conducted. The effect of vehicle lightweighting through downsizing and material substitution, as well as that of vehicle weight addition through new features and better acceleration, on the life-cycle GHG emissions from the U.S. light-duty fleet, is also explored.