I had a Durango 4.7 that couldn't break 14mpg and I didn't drive it that hard.
My current 2006 Tacoma doublecab 4X4 with a 6sp manual gets 17mpg city or highway and I short-shift and max air pressure in the tires.
The engineers for the 6sp manual hopefully lost their jobs....2400rpm at 70mph is ridiculous.
That 2400 @70mph is more of a function of your running gears vs. the tranny. I wish I had bigger gears in my truck for all the off highway I run. Truck was stock with 3.55 and I would like 4.10 but can't justify the $$$ to do it. I think overall I would get a little better mileage even going to the bigger gears because the truck would not work so hard in my predominant speed range. Highway mileage already sucks so that wouldn't matter.