One of my company's O/Os has a W9 with a 625 hp C15 cat, super 18, tall rubber and 3.08s!
I'ts only loping along at 1250 or so rpm at 70mph. Roadranger's website tells me it should be good for 115mph flat out at the governor. I think he says he gets low- to mid- 6s mileage wise, which is excellent for a non-aero truck.
On the other end of the spectrum, my 2007 Freightliner Columbia company truck has a 515 Detroit, straight 10, 11R-22.5 rubber, and 3.70 gears.
It's at about 1550 rpm at 70mph at the governer- right in the middle of the power vs. economy graph on Detroit's website. The drawback is that it is governed at 1800 rpm which doesn't leave much room for downshifting at highway speeds- 9th gear tops out at a little more than 60. I averaged 6.4 mpg in my identically spec'd '06 model, but the '07 was down in the mid 4 mpg (!) range when I got it at +/-8,000 miles. It has since freed up a bit ad in mid-to high 5s. I don't pussyfoot around -I certainly would pay more attention to fuel mielage if it wa my fuel nickel.
-p.