Not a big deal. My '03 already gets 20% better FE than a new truck, and it's still 90% clean.
My '89 VW Jetta diesel easily got into the 47-60 MPG zone, though I must admit its 52 HP was a little "light" at times. It also weighed 1000# less than a 2010 Jetta. This is where there is a lot of room for improved FE, IMO.
The only way big rigs are going to see any meaningful FE improvement is to roll back emission standards (get rid of EGR), and run at 55 MPH. I don't think that's a practical solution with cars routinely running at 70+ on the same road. There may be some benefit to aerodynamics and low RR tires, but they will offer an incremental improvement, not the final solution.
Attention: TDR Forum Junkies 
