Fidel Castro recently criticized Bush, saying that the proposal is impossible to achieve, and it will cost more money to fill the car with ecologic fuel than 10 poor persons eating out in a fancy restaurant.
I would say Castro is right. Food is expensive, no matter what. The only solution would be to extract energy from cellulose, which is the useless vegetation that most animals reject. Even the bacteria that convert greens into methane gas (methanobacteria) are picky with wood and thorny branches and leaves.
My preferred aproach would be to grow termites. Termites digest useless vegetation and can produce fuel by fermentation and passage thru chicken, worms or other animals. However, there is still some research to do.
In the meantime, I will starting to use a vehicle with gives 100 miles on 5 dollars of food. It speeds from 0 to 80 km per hour in a few seconds, produces no carbon monoxide and fixes itself when broken.
It has four legs instead of wheels.