How much weight could a plastic unibody save, and at what cost? The most radical polymer system is the Ultralite, a "concept car" based on carbon fiber composites that was developed by GM researchers given a mandate to obtain the highest possible gas mileage. The car, which was built by hand, incorporated a variety of weight- and fuel-saving technologies. Although the car was capable of getting more than 100 miles per gallon, it cannot be considered a prototype for a mass-market vehicle: it did not contain the space or safety features most consumers would consider essential and was never road- or crash-tested. Nevertheless, at 308 pounds, it represents the lightest auto body yet built of polymeric materials.

Although the Ultralite weighs about the same as an aluminum space frame, it would cost significantly more to produce in large volumes. At production volumes of 100,000, for instance, each Ultralite-style unibody would cost about $6,400. This estimate is based on the assumption that carbon fiber prices will remain at about $20 per pound. Proponents of polymeric materials have argued that the price of carbon fibers will decline as demand rises. But even if the price of carbon fibers fell to $5 per pound--a trend we do not foresee, since the production of carbon fibers is not necessarily amenable to economies of scale--the plastic unibody would still cost $3,500, compared with $2,500 for a steel unibody and $2,800 for an aluminum space frame at comparable production volumes. Moreover, at higher production volumes, the price of a steel or aluminum unibody will fall considerably, while the price of a polymer-intensive unibody will fall much less, making it an even less economically sound choice.