This design doesn’t appear to be affiliated with any major car company, but it’s a great idea: a new frame for the car that makes it a lot lighter. Lighter things take less energy to move (they have less inertia), so they use less gas to go the same distance, therefore the mileage is a lot better than heavier cars.
Check this out when you have time: Fiberforge makes an ultra light composite that can be used for car bodies:
http://video.google.com/videoplay?docid=4569577556800822039&q=amory+lovins