In designing vehicles with significant electric driving range, optimizing vehicle energy efficiency is a key requirement to maximize the limited energy capacity of the onboard electrochemical energy storage system. A critical factor in vehicle energy efficiency is the vehicle mass. Optimizing mass allows for the possibility of either increasing electric driving range with a constant level of electrochemical energy storage or holding the range constant while reducing the level of energy storage, thus reducing storage cost. In this paper, a methodology is outlined to study the tradeoff between the battery cost savings achieved by vehicle mass reduction for a constant electric driving range and the cost associated with lightweighting a vehicle. This methodology enables informed business decisions about the available engineering options for lightweighting early in the vehicle development process.The methodology was applied to a compact extended-range electric vehicle (EREV) concept. Detailed design and analysis was carried out to develop a vehicle concept which would achieve substantial vehicle mass reduction over the baseline design. This work resulted in the identification of many mass-reduction enablers and incorporated mass reduction potentials as well as cost estimates for implementing the enablers. In addition, a study was conducted to determine the cost of a battery as a function of vehicle mass for different generations of battery technology. Charts of the cost of lightweighting for all the enablers as well as the battery cost savings as a function of mass reduction were developed with and without the influence of mass decompounding. These charts were used to determine how much lightweighting is practical for various generations of battery technology.Results from the study indicate that there is generally a point at which the cumulative cost of mass-reduction enablers exceeds the benefit in battery cost savings. For early generations of battery technology where storage costs are relatively high, it makes more sense to spend money on vehicle lightweighting; however, as new battery technologies emerge (with lower cost and higher performance) the more costly vehicle lightweighting measures become uneconomical.