A framework for considering trade-offs between increased costs for lightweight materials and the attendant fuel savings accruing from weight reduction is developed in this paper.Two quantitative estimates of lifetime fuel savings due to weight reduction of a vehicle are derived. For substitution on an ongoing vehicle, available data suggest savings of about 0.26 gallons for the vehicle lifetime per lb. of weight reduction. When long-term changes in vehicles follow lightweight material feasibility, a range of estimates must be quoted; viz: 0.55 to 1.07 gallons for the vehicle life per primary pound saved.Considerations of trading these fuel savings against higher costs highlights two crucial issues: (1) the percentage weight reduction achieved in the substitution and (2) fabrication cost penalties. These technical issues largely determine whether a given application saves over-all resources. In a broad, approximate manner, the present trade-off analysis is found to be consistent with recent substitution practice and with apparent future trends.