"newer trucks are getting close to 9 mpg" - would you care to shed some light on which trucks are close to 9 mpg? Brand? Specs? Last time I checked, we were nowhere near that number with 80000 lbs gross running the east cost at 65-70 mph. 5.9 mpg is low, but it's actually pretty close especially in the winter.
Just curious, that's all....
Mine do. 2015 and newer Freightliner Cascadia's with the all-Detroit drive line. DD15's @450 hp. DT-12 auto-shift transmissions w/direct drive. Detroit rear ends with 2.7?:1 ratios. We tried some with the overdrive transmission, but they get a bit worse fuel mileage.
General takeaways ...
Detroit drive lines are the way to go.
Body styles make a difference. Freightliners get better fuel mileage than Western Stars do.
Getting 9 mpg (Imperial) w/80,000 lbs gross or less is pretty easy.
Getting better than 8 mpg (Imperial) w/107,000 lbs gross is tricky, but can be done.
Using a manual transmission, even with the best driver, will cost you 1.0 to 1.25 mpg (Imperial) over the DT12.
There is roughly 1.0 mpg difference between summer fuel and winter fuel.
I have some guys that run super light that get 10.0+ mpg (Imperial).
Over 60 mph sucks fuel no matter what drive train you employ.
DO NOT put an old school manual driver in a new technology truck with a manual transmission unless that driver has a "come-to-Jesus" moment vis-a-vis new technology. Your fuel consumption will be sub 6 mpg (Imperial) even with a Gen 4 or Gen 5 Detroit.
As a testament to the value in Detroit drive lines, I have a 2021 Freightliner Cascadia Evolution (one of the last of that model built), built for heavy haul, with a Detroit DD16 @ 600 HP and 2,050 ft/lbs of torque, and a 13 double over manual transmission, run by a driver very knowledgeable in new engine technology that gets 8.0 mpg steadily.