This thing is huge and it weighs a lot, not much is going to change that.
I made sure to pick out a 2 wheel drive, 6.2L diesel engine, with the TH700R4 automatic transmission and 3.08 gears.
I also burn used vegetable oil.
Turns out I do not have the 3.08 rear diff gears I thought I had. At some point they were swapped for 3.42s.
One of my first ecomodds I did was installing better and cheaper fuel filters so that I could burn waste vegetable oil. Soon after I was remaking the air intake and installing a ram air intake. Next I changed out the belt driven fan for 3 electrics. This was all back in 2006. That setup was good for 25-26mpg high way.
In 2007 I redid the exhaust, not really for economy.
In 2008 I switched from EGR "c-code" intake manifold to a "j-code" non-EGR.
That configuration turned fairly consistant 27mpg any time I got out on the highway.
Over all a nice improvement from the 21mpg highway I started with.
In late 2009 I tore the engine down for what should have been an overhaul that took 2-3 months max and found the block had started to crack in 3 places.
13 months and about $1800 later the new engine was in.
After I built it in my living room.
I up graded to a 6.5L block and kept my "small valve" #162 casting heads that a known for economy, not power.
6.2L heads on a 6.5L engine block bumps the compression to about 22:1.
I also found the time to file fit piston rings to reduce blow by, ported and polished the intake and exhaust port and ported the intake manifold.
During the engine build I installed provisions to run an electric water pump, not fully sure if it would work or even fit. So far it has.
I also converted over to electric fuel pumps.
Runs smooth, burns no oil and leaks no oil. (a huge improvement)
Smooth wheel covers.
Ram air fed, Twin turbochared (and intercooled) with a Garrett T76 feeding a holset HE351VE.
3 inch down pipe into a 4 inch weed burner straight pipe.
Water injection during the summer.
Installed 3 solar panels.
Installed LEDs every where but behind the dash and dont use LED head lights.
Add more Solar panels.
Peterbilt bumper and air dam.
Install 3.23 gears with an eaton posi. The Eaton positrac isn't eco, just bad ass.