Isn't the frame itself something that could be made of aluminum to substantially reduce weight? I once made an aluminum frame for an electric vehicle that we raced in high school endurance races. The entire frame could easily be picked up with 1 hand (best guess is 15lbs).
Reducing weight is one of those things that allows you to further reduce weight exponentially. A lighter car doesn't need such beefy suspension, axles, brakes, etc. It can have a smaller and lighter engine since it accelerates easier. Get the engine small enough and it will operate closer to peak efficiency on the freeway as you have to give it a greater percentage of throttle to overcome aerodynamic drag.
For coastal or snowy areas, an aluminum car would be impervious to the corrosive environment.
How much extra would it cost to build a conventional frame car out of aluminum? Lotus uses an aluminum chassis in the Elise as shown here.
