The best way to reduce the oil consumption and thereby increasing the efficiency is to use light weight material. Carbon fiber is a material, having good strength and is light in weight when compared to material which is presently used in automobile industry (two times stronger and two third less in weight than steel).

Carbon fibers are thin strands of carbon which can be bound together and then molded to any shape. Replacing this with the current steel can reduce the weight of the automobile by nearly 60%, this in turn  reduces the fuel consumption by 30%. This is quite a good in fuel saving. The other benefit of using this carbon fiber is that it reduces the emissions by 10% to 20%.

This is what wikipedia has to say about carbon fibers:

Carbon fiber (alternately called graphite fiber or graphite fiber) is a material consisting of extremely thin fibers about 0.0002–0.0004 inches (0.005–0.010 mm) in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in microscopic crystals that are more or less aligned parallel to the long axis of the fiber. The crystal alignment makes the fiber incredibly strong for its size.