[oil pan 4]

Back in 2006 I was looking for an old 2 wheel drive 6.2L diesel chevy blazer to be my home improvement and work truck, road trip, medium duty towing and mild off road machine. I knew $3/gal fuel prices were here to stay and I had to do with out certian things to get good fuel economy (like no 4wd, no huge tires, and slow to get up and go).
I had always heard diesel chevy blazers get very abnormally high fuel economy for a "SUV". Lengend had it the after market (banks engineering) turbocharged 5-speed versions were getting nearly 30mpg highway, but they are very rare.
Really who would sell a truck that gets up to 30mpg highway?
I looked high and low for a chevy diesel and setteled on a 6.2L diesel suburban with a TH700 (the fuel economy transmission). It turns out my truck also had 3.08 rear end gears too.
EDIT: the build sheet says it has 3.08 gears but the axle had been swapped for a 3.42 gear axle at some point before I bought it.
In stock form it got an impressive 15mpg city and 20mpg highway. Looking under the hood I knew there was lots of room for improvement.

First thing I did was change the air filter, motor oil and diff oil to synthetic. That didn't quite give me +1mpg but I started putting up 21mpg Highway for the first time.
I started off the real mods by tossing the very restrictive warm air intake and hand building a ram air intake set up. I could tell this was a choke point.
That netted gains every where, low end and high end power, fuel economy a clear +2mpg gain and increase in over all driveability. I also made it so the ram air could be pulled and almost instantly converted into a warm air intake for improved operation in the bitter cold.
Several month later changed tires, slightly taller. No change in MPG that I could see. I expected it to go down.
My second big mod was tossing the stock 5 blade 21 inch belt driven clutch fan and replacing it with "universial fit" electric fans.
The fan swap netted me nothing but gains also: Engine was quieter, smoother running, cranked over faster, warmed up sooner in cool weather and produced a 2.2mpg gain in combind city and highway driving.
At this point the big bad suburban was putting up solid 25mpg highway MPG numbers.It stayed in this configuration the longest.

Next big mod was I upgraded to 2.5 inch free flowing exhaust. Suprizingly no MPG improvement that I could clearly see. I didn't expect to see much anyway. Only gains to be had were with high end power and 100'F or so reduction in peak EGTs.
The final mod before the insanity started was I installed the more open and free flowing non-EGR factory "J-code" intake manifold.
Picked up 1MPG for $60 and a 45 minute install time. (I wish they were all so easy).
With the external engine mods done I was able to get 17mpg city and 26/27mpg highway.