Let's get this straight. Barack Obama DOES NOT have a plan to increase fuel efficiency in automobiles.
Barack Obama knows jack squat about designing cars. He does, apparently, know how to use the coercive powers of government (i.e. point guns at people and say, "do it"), and that's what he's doing. His plan is to threaten those who do engineer vehicles.
The thing is, if it was so easy to do (scientifically and economically), then the automakers would be doing it on their own. If consumers really demanded it, then the automakers would be working toward it.