Toyota's Prius is already, almost, the most fuel efficient car on the planet. The green heads in CA where I live will pound you to death with their eco-clubs if you try to argue otherwise. Other cars do get better mileage…they just don't want to be confused with the facts.

Japan's other three want to challenge the Prius. 

1. Nissan thinks it can roll out a gasoline only car in two years that will get almost the same mileage as the Prius Hybrid.

Nissan will use direct-fuel injection to boost fuel efficiency and a turbocharger to reduce pumping loss. Nissan will also reduce energy loss through friction and heat by coating all areas with a thin layer of diamond-like carbon.

2. Honda has a technoloy that allows its V-6 engines to operate on only 3 cylinders at high cruising speeds and on 4 cylinders when slowly accelerating - improving fuel efficiency by 8-12%.

3. Finally, Mazda will emply Smart Idling Stop System - SISSY for long - in cars debuting in 2009. Cars will shut off when stopped at traffic lights and so on. Fuel consumption will be reduced by 10%.

And, if we were to add these three methods together, what would we have?

Japanese are already the best carmakers. Add to that even better fuel efficiency and America's Big Three will continue to be mired in restructuring. 

source