LAS VEGAS, NV (Autodesk University), Dec 2, 2009 - We all know a lighter vehicle will save fuel since it is easier to accelerate. F=m*a. The force required is proportional to the mass for a given acceleration. But as Amory Lovins of RMI points out in his AU keynote, it's better than that.
Say you are able to go use a carbon fiber frame instead of steel, and save a few hunded pounds. That would immediately result in fuel savings. But now factor in that the engine can be smaller. With the engine smaller, a smaller hood and fenders will be required. The brakes may be downsized, maybe the transmission. Each pound saved in the frame may be worth 1.5 pounds off the car.
While electric cars may add battery weight, they might more than compensate by losing a differential, a transmission, a gas tank, etc.
Such indirect savings also come into play in architecture. Superwindows that block solar heat heat but insulate may cost more in a retrofit but may result in a total savings because the peak air conditioning load is reduced, requiring smaller compressors -- or sometimes even eliminating them.