Some answers from what i've found.
Hydrogen-generators take more energy to make hydrogen than what you will get back from them.
Synthetic fluids, motor oil, power steering fluid, ATF
, rear axle lube: they last a long time but offer no measureable improvement in MPG
Low rolling resistance tires do help. Not much data on just which ones are low rolling resistance. A green outfit did test a few tires in our size range and found the BFG Long Trail to be the best with the Michilin XP rib right behind. That data is seven years old, though. I do think the OEM Firebombs are pretty good. One surrogate paramenter for rolling resistance is noise. Quiet tires seem to have lower rolling resistance. The Firebombs are quiet and you can fearlessly over-inflate them.
Adjusting the nut behind the wheel (the driver).
Now, you are up to speed.
tire wise, I pulled off a very tired set of Goodyear Silent Armors and replaced them with Wrangler SR/A's (very similar to Michelin LTX)..
as with any new tire, the increase in tread flex over the same tire worn out generally produces a decrease in fuel economy, so I was pleased when it actually increased .25 mpg
(but was it worth laying out just as much as a set of S/A's in the middle of winter? Hell, no. I chalk that up as one of my classic theory on paper does not equal results moments)
as far as the loose nut between the wheel & the seat, I've found that on the expressway, 62mph produces the best result in clean air, 65mph behind a semi (52ft box trailer) is managable. Don't follow too close, I get in just close enough for the wind to quit whistling across the CB antenna (mounted in the dead center of "X" drawn between opposite corners of opposit bumpers). I've watched the overhead lie-o-meter increase 2 full mpg
on a 50 mile run..
also, with my truck outfitted with a chip (DPTuner) in economy mode, about 2,200 rpms is about all you wanna give it. Keep the smoke IN the tailpipe.
I have a hard time believing that 4x4
SuperD's are capable of anything much over about 25.. 2wd's maybe 30 (due to lower drag from the lack of all that BS front axle, lower body/hood lines and less weight to haul around)
tuning a gas motor for fuel economy I can do.. but with only two ways to alter how much fuel a diesel consumes (how much it gets and when it gets it), I'm a pure noob...
tell me about the snowplow on the red truck in your avatar.. sheet metal? UHMW plastic?
next time your AC
goes on the frtiz, hit me up. I specialize in AC