Just checking in again, it's been about a year since I started driving 55 on my 2009 Toyota Corolla.  I'm still getting about 40 MPH combined on an EPA Hwy estimate of 30 MPH.  An extra benefit that hadn't occurred to me until I took the car for it's 30K maintenance is the reduced wear and tear.  My brakes were still in great shape probably because I seldom have a need them while I'm on the highway.  I'm sure other parts of the car are benefiting from the lower stress driving.
As for tailgaters, I drive on the far right lane at times when traffic is light. If they can't figure a way around me with two open lanes to the left, I guess I can help them save some gas.

Tin Cheung