In the same way that the first X Prize helped launch manned spaceflight, the *automotive* X prize is designed to encourage the development of cars that get over 100mpg.
The rules are very complex, but I saw a talk tonight by a guy who has basically read all the rules and investigated all the entrants. The design ideas range from clever to wacky. Have a look:
http://www.xprizecars.com/ He also mentioned that you can get far better mileage out of cars by reshaping them to get a better drag coefficient. This guy made a 1992 Honda Civic get close to 100mpg just by adding some extra aerodynamic pieces.
It's fantastically ugly, but it's what we engineers call a "proof of concept". Most of the gasoline at high speeds goes to fighting air friction, so if you can reduce the drag coefficient as this guy did, it makes a huge difference.
What stuns me is how little work it took to raise the mpg through the roof. If the automakers wanted to, they could easily make high fuel efficiency vehicles.