I have herd that too: "the [any engine] is most efficient between [always some number between 1800 and 2200]RPMs"
I don't know maybe it gives the peak for fuel economy and power, that could be what they mean for "most efficient".
I also noticed EVERY one that says that is getting well under 20mpg.
Trust me they say the same thing over on dieselplace.com on the 6.2L and 6.5T forums.
The guys with the 4x4 6.5T turning 1800 on the highway, with a non-over drive transmission on 4.10 gears are only getting about 15mpg.
I have adoped nearly the same setup as big dave.
Diesel engine, with a trans that has a lock up converter and a .72 over drive (makes it act a lot like a standard trans), with 3.08 rear gears and 31'' tall tires.
My truck chugs along at 1250RPMs @ 55mph and 1350RPMs @65mph and it got 26 to 27mph with out drafting or hypermiling.
I can get over 30 when drafting. The first time I drafted for more than 10 or 20 miles I went for nearly 400 miles and pulled off the high way to fill up convinced my fuel gauge was broken, nope I was just getting a little over 31mpg drafting off a big uhaul going about 55mph.
What people are saying is best and what we test are 2 differet things.
I say go with the tried and tested set ups and stay away from the old wives tails.