Solar Panels need to be improved to harness more energy. Right now there is an effort to swap out the components and elements in a solar panel to increase efficiency. So far in small test samples researchers have been able to vastly improve the efficacy. Of course this does not mean that solar energy is the all around solution, rather it is just one of many. What would be nice to see is the price of solar panels reduced so that an increasing number of people can put them on top of their houses, thereby eliminating the need for more land.
As for the automobile industry, well there are other ways of increasing fuel efficiency other than weight, but you are right: it is a huge factor. Light and cheap can still be safe though. Using better design, engineered more for performance and less for aesthetics, structural elements can vastly improve the safety of a car. Anywho, this is likely never going to happen because of the vested interests in the powers that be. The good old status quo.
As for that carbuerator, I think PK only said 40 MPG, not that I have any take on the story either way, just clarifying.