The competition among materials is evident in the composition of the U.S auto. In 1978 the average U.S auto weighed 1800 kg and consisted of about 60 percent cast iron and steel, 10 to 20 percent plastics, and 3 to 5 percent aluminium. The 1985 U.S auto comparison weighed an average of 1400 kg and consisted of 50 to 60 percent cast iron and steel, 10 to 20 percent plastics and 5 to 10 percent aluminium. Thus in the period 1978-1985 the percentage of steel declined, that of polymers increased, and that of aluminium remained about the same. In 1997 the domestic U.S.auto weighed an average of 1476 kg, and plastics comprised about 7.4 percent of it figure (1.15) . The trend in the usage of materials appears to be more aluminium and steel in autos and less cast iron. The amount of plastics (by percentage ) in autos appears to be about the same (figure 1.16).