Poor Fuel Mileage

Looking for pointers here. I have an 06 Columbia with a 60 series Detroit. 12.7. I'm getting between 4.8 and 5.1 mpg. Local work. Home everyday so no overnight idling.

So far....... Overhead just done, tires are up, new exhaust manifold with new gaskets. Also has 14l injectors installed but the pulse time is cut back so it's not dumping fuel. It's tuned to about 550ish. Truck holds 35-40 psi under full boost. Runs and pulls great. Just can't get the mpg.

Today new air cleaner and Donaldson m100463 muffler installed as I've seen it recommended here more than once.

I'm asking before I run the new muffler because the one I removed was a straight through design but is different than the Donaldson. Looked for a part number on the old one but no luck. Is it possible that even with a straight through design it would still have a decreased flow rate?
I'm about out of ideas here. Any help is greatly appreciated.

Thanks in advance.

 

Truck does have an upgraded turbo and does not have a hard bend directly off the turbo. Egt is all but impossible to hit 750-800 in the dead of summer. Truck was purchased at a sale and has about 250k on an overhaul. That was done after it was purchased. Its not someone else's promise. Don't believe I've ever gotten 6 mpg since the day I got it.

 

Not sure on rail pressure. Truck does smoke some after the tune. If no other self check ideas come along I'll schedule with a local shop to have that checked out as I have no way to do that here.

 

11r24.5 with 370 rears I believe with a 13 speed Eaton. Runs about 1400-1500 rpm at 65-70 mph.
I
have spoken with a few local guys with similar set ups and they claim 6 or so. Even the shop that did the in frame thinks it's low but they only do engine work. That's why I've moved on to things that may help what I feel is an issue.


Everything that has been done so far were needed repairs anyway so I did some homework on the better parts to help mpg. Now that everything has been fixed I'm to a point I will be throwing money at something. That's why I came to the professionals. Lots of smart and experienced people around here from what I've seen.

 

That seems to be the story across the board with the exception of mine. I guess maybe I could check the charge air cooler for a leak but I would guess I'd see a loss of boost pressure or more heat with a leak. I'm just out of ideas now.