What will the fuel that will be used in future?  

well I saw a car once that was being developed by a university to run on solar power. It didn't really seem feasible because the car was built wide and flat on top (like a hang glider) so it could get enough light to power the motor.

I personally thinkwe will have to develop something like at the end of back to the future where the car processes garbage and waste and makes energy out of it. It might not be too clean for the air, but it will take care of our waste problem and fuel problem.

well i think in addition to solar energy it is bio diesel which will go a long way.
it works as efficent as petrol and diesel. and u know what?
u need not to modify your engine to run on bio diesel.
if u r from india must be knowing that the chief minister of chaatisgarh also run his vehicle on bio diesel.