Use phase fuel consumption is responsible for the majority of an automobile's life cycle energy consumption and greenhouse gas (GHG) emissions. Lightweighting is an important strategy to reduce use phase fuel consumption and potentially reduce vehicle life cycle impacts. A popular lightweighting technique is material substitution, in which conventional materials (e.g., iron, steel) are replaced with lighter ones (e.g., aluminum, magnesium). Material substitution, however, often results in higher material production impacts. A life cycle approach is useful in evaluating these material tradeoffs and assessing the overall energy and emissions benefits of lightweighting technologies. Thin-wall ductile cast iron (TWDCI) is a lightweighting fabrication technology that can provide comparable weight reduction to aluminum while having better mechanical properties. This study develops a parametric life cycle model to assess the life cycle performance of TWDCI compared to conventional cast iron and cast aluminum in terms of energy (MJ) and GHGs (as carbon dioxide equivalents - kg CO2e). This model was applied to three lightweighting cases: a differential casing, an engine block, and replacement of all cast iron parts. Fuel reduction values (FRVs) are used to calculate change in fuel consumption due to lightweighting. A sensitivity analysis on these lightweighting cases is employed to determine the mass reduction required to achieve net life cycle benefits and to show the effect of alloy composition on life cycle energy and emissions. Lightweighting by 2% results in equal life cycle energy and GHGs for TWDCI and conventional cast iron while 37% lightweighting is required for TWDCI to equal cast aluminum impacts. The implications of powertrain resizing afforded by lightweighting are also explored.