I've been doing the 55 mph routine long before I found this site. I've owned hybrid cars with the built in mileage computers, and had made it a game to push the graphics ever higher, and log ever higher figures into my sheets. I no longer have the hybrid cars, but still keep to the 55 mph rule. Case in point, my current ride, which is a 2008 Chevy Impala 2LT with the 3.9L V6 that cuts down to three cyilnders under low load conditions. Tonight on the way home, I've posted a new high number - 33.8 mpg by the car's computer. This isn't via a solid nonstop highway drive either. I have about 15 traffic lights on my 50 mile commute to get through, and three of them still managed to get me to come to a full stop, despite the late night hour that is my drive time home. I have a photo of the car's mileage computer as proof: 
Bryan Foust
Paradise, TX
EPA MPG rating on this car is 18 mpg city, 29 mpg highway, 22 mpg combined
http://www.fueleconomy.gov/feg/noframes/24696.shtml
Update, 10/14/2008
I am writing tonight to give an update on my efforts to push my fuel economy to new heights on my 2008 Impala. Tonight, not only did I beat my old record of 33.8, I practically shattered it. I pulled into my driveway after my drive home from work, to find the mileage readout posting a new high of 34.9 mpg. That is not all, for a while, it was showing as high as 35.3 mpg, before the hills in my immediate area took their toll on the numbers. As with my first email, I have a photo of the screen:
I don't care that the price of gas has been coming down as of late.
Maintaining my rule of driving no faster than 55 mph, combined with the
recent downward trend in prices at the pump (I will not call it "cheap"
untill we see pre-Katrina prices, and I'm not holding my breath for
that) both combine to further help give my fuel budget a bit of a break.
Bryan
Update January 4, 2009
Once again, I write back to share another experience of keepting to a
maximum speed of 55 mph. I recently just returned from a holiday visit
to family over the Christmas and New Year's holiday. The drive up to
Colorado included a battle with headwinds, and resulted in a none too
remarkable 32.3 and 31.7 mpg respectively out of the two tanks of gas
purchased for the approximately 750 mile drive.
My drive home however, would prove to be much more fruitful. The temps
were warm (almost too warm) and winds more favorable for the return
trip. I topped off the tank before heading down the highway, and set
the cruise to the requisite 55 mph speed. As the miles ticked by, the
computer kept gradually climbing higher, as the "AFM" system in my
Impala took every possible opportunity to turn the V6 engine into a V3.
By the time I entered into the Texas panhandle area out of New Mexico,
the mileage computer was solidly locked into the 36 mpg figure. Still
the computer would notch upwards a couple of more ticks by the time I
began to see Amarillo on the horizon. Needless to say, I was thrilled
to see my full size car enter into fuel economy territory occupied by
the Chevy Aveo. As Amarillo was my planned city to stop for gas, I
pulled into the Pilot Truckstop located at I-40 and Loop 335. When I
shut off the ignition, the computer was showing 36.5 mpg. As with all
of my record mileage claims, I've included a picture of the computer:
http://farm2.static.flickr.
This number was obtained after a 437 mile drive.
The rest of my drive home would still prove the merits of keeping my
speed to 55, and letting the car's cylinder deactivation make the most
of itself. The continuing drive down US 287 to the DFW area includes
lots of little towns to slow down through. The mileage computer still
posted a very respectable 35 mpg, despite having traffic lights to
contend with in the small towns.