Even if cars soon start running entirely on electricity or hydrogen, they'll still need 100 gallons or more of oil to make their plastic parts, such as seats, dashboards, bumpers, and engine components. And some day that plastic may be recycled back into fuel.Cars of old were mostly steel, but the use of lightweight alternatives has dramatically increased in the last couple of decades. Whereas almost no plastic could be found on a car from the 1950s, today's automobiles have more than 260 pounds (120 kilograms) of plastic on board, according to the Transportation Energy Data Book."It is expected that high oil prices and strict CO2 standards will accelerate the growth [in plastic use]," says Aafko Schanssema from PlasticsEurope, a plastic industry group based in Belgium.Plastics improve fuel economy by reducing weight, but they also require petroleum as a raw ingredient. "Plastics are in fact solidified oil," Schanssema explained.Although different plastics have different recipes, it takes roughly 0.4 gallons of crude oil to make 1 pound of plastic. Globally, around 8 percent of the oil that comes out of the ground is used to make plastic.Car dietThe average car is a mix of materials: glass windows, rubber tires, lead batteries, copper wires, as well as traces of zinc, magnesium, tin, platinum and cobalt.However, steel is still the single most important material in cars. It is strong, durable and malleable. On the flip side, though, it is relatively heavy. For this reason, car manufacturers have been trimming down on its use.For domestic cars, the percentage of weight in steel and iron has dropped from 75 percent in 1977 to 63 percent in 2004, according to the Department of Energy's Transportation Energy Data Book.