Tec is right, Lorries cruise at 56 for similar reasons. It's the most speed you can attain before the air gets too thick and the extra distance you cover is disproportional to the drag created.
For me, I can manage 35mpg if doing 55-56mph (on sat nav, about 57mph, 58mph on the cruise). If I draft as big a lorry as I can find, I can eek it up to 39mpg. (obviously getting too close gives far better mpg but is stupid).
If I drive a proper speed, 70~80, then I get about 28mpg.
Recently I did an 850 mile trip in my CLS 350d , averaged 44 mpg for the entire trip.
First 425 mile leg was done without stopping , return leg there was one stop involved - for diesel.
Average speed was around 60 ish mph IIRC for the entire trip.