generally higher costs. A comparison of some of these properties for various alternative
automotive materials is provided in table 3-2.Weight
Weight is a primary determinant of such critical vehicle characteristics as acceleration, handling,
fuel economy, and safety performance. According to one estimate, 75 percent of a vehicle’s fuel
efficiency depends on factors related to weight, with the remaining 25 percent dependent on the
vehicle’s air resistance.^12 For a typical vehicle with an internal combustion engine, a 10 percent
reduction in weight results in a composite (city/highway) fuel savings of 6.2 percent.^13
In the future, the substitution of new, lightweight materials for steel holds the promise of
making vehicles lighter without sacrificing size and comfort for passengers. Table 3-1 gives
estimates of possible weight savings compared with steel using various alternative materials. On
an equivalent part basis, relative to carbon steel, high-strength steel saves 10 percent, glass FRP
25 to 35 percent, aluminum 40 to 50 percent, and graphite FRP saves 55 percent. On an entire
vehicle basis, maximum practical weight savings are about two-thirds of these values, because
only a fraction of components are candidates for substitution.^14
Weight reductions in primary vehicle components also enables secondary weight savings in the
supporting subsystems. For example, the engine, suspension, and brake subsystems can be
downsized for lighter vehicles, because their performance requirements decrease as the total
weight of the vehicle drops. The ratio of secondary to primary weight savings can be estimated
only roughly, but a general rule of thumb is that about 0.5 pounds of secondary weight reduction
can be achieved for each pound of primary weight removed, provided the secondary subsystems
are redesigned.
When coupled with a smaller, fuel-efficient powertrain, these weight savings can be used to
make vehicles more fuel efficient and environmentally friendly. Alternatively, the weight savings
could be used primarily to obtain increased performance (e.g., increased horsepower to weight
ratio) or to offset weight increases in other parts of the car so as to maintain compliance with
environmental regulations. The market continues to pull vehicles in the direction of larger sizes,
shorter O to 60 times, and so forth, with the result that the average horsepower to weight ratio of
new cars has been increasing every year. This suggests that the use of lighter weight materials to
achieve higher fuel economy will only occur if the market values fuel economy more highly than
acceleration performance, or if the market is pushed in this direction by policies such as higher gas
taxes and Corporate Average Fuel Economy standards.
As long as lighter weight materials carry a cost premium that cannot be recouped by the
customer through fuel savings, substitution will tend to occur in vehicles in the luxury or high-
performance class (e.g., the Honda NSX and the Audi A8) where customers are willing to pay
more for the better acceleration and handling characteristics of a lighter car.
(^12) Audi, "The New Mobility Enterprise: Revolutionary Automobile Technology by Audi,” brochure, August 1993, p. 6.
(^13) Energy and Environmental Analysis, "Documentation of Attributes of Technologies To Improve Automotive Fuel
Economy,” report prepared
for Martin Marietta Energy Systems, Oak Ridge, TN, October 1991, p. 2-16. 14
Assuming comparable size and interior room.