I think it is rather easy to see how a central algorithm would be much better at a) maintaining a safe distance from the car in front, and b) merging quickly and fairly.
Route selection is just one of the things an algorithm could do.
Traffic, by Tom Vanderbilt, has many interesting things to say on the subject: http://www.amazon.com/Traffic-Drive-What-Says-About/dp/03072...
Lower inter-car distance on highways will also mean lower air resistance, and hence, higher mpg.
I doubt that the net effect on fuel consumption will be substantial, though. Chances are that people will want their driverless cars to drive faster, so that they can live farther away from work. IMO, the only thing that will help there is pricing (fuel, road use, etc)