I've got a 98 12v CC LB Cummins w/an Automatic & 33" BFG Mud T/As. Last fall I had the tranny built up & now I've finally gotten around to working under the hood. first in line was a set of autometer gauges (pyro, tranny, boost), then slid the fuel plate fwd, turned up the boost & some afc tuning. I have to guess on everything but the gauges, bc my buddy the diesel mechanic was doing that while I was hooking up the gauges.

Anyway, here are the results: a lot more usable mid range power & now the truck pulls longer. Cody set it to start defueling @ 2600rpm - this made a HUGE difference.

Living where I do, there's not much flat land, but here's what I've been able to notice in 1/2 a days worth of driving around.

cruising @ 60mph (1500rpm) = ~2lb boost & about 400 degress on the pyro
cruising @ 70mph (1700rpm) = ~5lb boost & 500 degrees on the pyro
if I really stomp on it I can get 20psi of boost & max temp seems to be around 800 degrees.
If I stomp on it, the boost gauge starts moving around 1200 RPM

I know more power can be had, but considering this vehicle spends a lot of time off road & on boat ramps; I'm going to leave it here for now.

I am curious, is there a correlation between boost levels & fuel economy? Is this a good excuse to go with 35s when my 33s wear out?

 

800* sounds like your thermocouple is installed post turbo. Or, you can pour WAAAAAY more fuel to it.

Also, try an adjustable boost elbow so you can get more pressure before your WG opens.

If you're making boost, you're under load, so you're burning fuel. More boost, more load, more fuel. Roughly speaking.

 

Pyro must be post turbo, and your boost sounds low to me. Stock turbo, I'd run 7-10 psi of boost on the highway, and average about 700 degrees constant speed flat ground.