Long time subsciber, first time poster!!
The only thing left out of the discussion about the affects of faster travel speed... is the simple fact that most of these vehicles are geared to run somewhere well above the optimum 1800 rpm and as you increase travel speed you also increase engine speed. Since the engine efficiency drops off quickly as engine speed increases above 1800 rpm, the faster travel speeds really hurt. A simple test would be to compare fuel economy at the same engine rpm in direct vs. overdrive. That would show you the areo and friction stuff, excluding the engine and some of the transmision. I have lots of mpg data on a stock 1995 1/2 ton 4wd 6.5 turbo with 3.42's. I couldn't tell you what the mileage is at 65 mph, I've never done that! My year around average is about 14.5 mpg. Less when towing (11 ish) and more when not towing, especially at 70-75 mph (16.5 ish). Enuf for my first post.
1995 GMC Sierra 6.5L K1500 Ext Cab Short Box, 316,000 miles, Kennedy Heart Beat @ 233k miles (21:1), GM Dual Thermostats - Housing - High Flow Water Pump, 9-Blade Steel Fan, Kennedy Diesel Special Calibration Fan Clutch, Kennedy Diesel Exhaust - 3" Downpipe - 3.5" Pipe - See Thru Muffler, DSG Cam Phazer, Kennedy Remote FSD Behind Drivers Side Headlight, A-Team Turbo, Kennedy Diesel Fuel Economy Programming, A-Pillar EGT, Boost & Tranny Temp, GU6 3.42 & 245-75-16 Rubber, Stock GM Flat Air Filter & Box, Zero to 60 mph in 9.5 seconds on Wintermaster.