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.