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I absolutely concur regarding the 2000 rpm.
Last year, I traveled from Denver to El Paso, and back in my F-250, as part of a military convoy (in GSA vans) so we were restricted to 55-60 mph on the highway per gov't regulations. I got 21-22 mpg on this 1200 mile trip. On different "non-military" trips of similar distance and terrain when I traveled a more reasonable 65-70 mph my mileage drops off to 18-19 mpg. In a couple of instances when I had a particularly heavy foot and averaged more like 75-80 mph, the mileage really drops off. I have wanted to plot this on a graph but haven't got around to it, but I think the mileage drops off exponentially at speed above 55 mph. One key is keeping an eye on the turbo psi - if the turbo psi starts to rise, your fuel economy is going to drop. Under no-to light-loads (i.e. not accelerating, not going up a hill, not towing, etc), the turbo psi is a essentially 0 below 2000 rpm.
2005 F-250 CC, 6.0 L, 6-speed manual, 4x4, 90K miles
2007 Ford Freestyle Ltd, 30K miles
2006 Kawasaki KLR-650, 6K miles
1995 Kasasaki Concours (ZG-1000), 30K miles
1982 Jeep CJ-5, 133K miles