Improving gas mileage for cars and trucks is a goal shared by virtually all environmentalists — but it turns out that efforts to do so have a hidden downside. As vehicle manufactures seek to improve fuel efficiency by making their products ever-lighter, an increasing percentage of cars and trucks are made from plastic. Unlike the metal components traditionally used in cars and trucks, the plastic is difficult to recycle. Moreover, it is hard to resell at a profit, because it is cheaper to make plastics from raw materials than recycled ones. “There’s no really economical way to either separate it out, or there’s no vendor who would want that material, so off to the landfill it goes,” said Bob Garino of the Institute of Scrap Recycling Industries. With more than 10 million trucks and cars consigned to the junkyard each year, that’s a lot of plastic waste. Enviros say the solutions lie in improving separation techniques for recycling and in designing cars with an eye toward their end-of-life uses.