Help with my MPG before i throw in the towel

a winter fuel will give you a bit less mpg than the summer fuel--wether it is the fuel or just conditions--it seems normal for a lower mpg in the winter months
but yes you hqave to lower that rpm down probabely at least 3oo rpm,not sure if it is worth it for you to change differentials --or else slow down to 60 mph and see if it helps

 

the winterfuel and the cold weather is bad for mpg,i change my driving and iam right now ok ,150k @5.9 my last trip cali -florida pass +6 WITH 36K IN THE TRL ,speed @68, after changing the mufflers and without speedlimiter and without cruise iam ok ,when iam not heavy +6,5 TO 7 WITH CAT C13 -500HP 1750 TORQUE,the ecm tuning was were helpfull for the hills

 

and when the hotweather come i hope for a increase from my mpg

 

Check the charge air cooler, if it is leaking you will lose mpg

 

Mismatched tires is a big part of your problem. Your draggin the smaller diameter tires down the road...

 

You need to figure or check what gear ratio the rear ends and whether or not the tranny is an overdrive. You might have a truck that was spec'd to pull heavy loads and not go fast in hilly terrain, and really isn't spec'd for running coast to coast.
Winter fuel, and everything being colder and thicker can cost you around 1mpg vs summer time. The air dryer cycling means the compressor is running which is more drag on the engine, have you replaced the cartridge? It may not be an actual air leak, they will cycle like that if they need to be serviced.
Putting on the lo pro's increased your engine rpms at any given actaul speed vs the older 11r tires, but low pro's have slightly lower rolling resistance, so the engine speed, which is determined by the rear ends is the deciding factor.

 

I'm no expert, buy my guess is the tire size coupled with gear ratio. I would imagine that by using the "big rubber" that was originally spec'd for your truck, you'll be better off by effectively lowering your operating RPM.

 

I doubt that changing tire size will make much differance in fuel mpg. Same with the FASS it helps but wont pay for itself in fuel savings. Getting the ecm tuned would be the best money spent. While it is expensive it should pay for itself in a few months. I would stay away from Pittsburgh Power. I have a PDI tune in my cat, very happy with it, but I am not sure how they would do with a Detroit.