They have (almost) already done that!
I've gotten my 2.5L up to 33MPG on the highway. And if you stripped it, you could probably get 35+ MPG.
I would dare to say that the most wasteful use of gas on the highway is used while accelerating. Cruising at a lower RPM might help on long trips, but I think the best way to save on the highway everyday is to stop enjoying the onramp and revving to 6000k.
Although, this hasn't stopped me, yet!
