Ok my buddies new to him 05 chevy 6.6/Allison is having issues. Seems to drive fine with the exception of fuel mileage being poor. Now hitched up to his empty 5er he can only mange 40mph up a good hill with a full head of steam. My 06 Dodge can pull them same hill with our 5er loaded up at freeway speeds, I have even had to pull this hill from a dead stop at the the bottom and can still manage freeway speeds by half way. I turn to you to make me look good so I can give my buddy some place to look. He has had the injectors rebuilt and the fuel pump regulator replaced. I already told him to give up and buy a Cummins

 

There have been some Duramax trucks that have worn holes in IC piping that rests on frame on drivers side, kinda near the Steering box. every hard to see, almost have to feel or remove the hard metal pipe. That could cause boost leak.
Only other thing I can think of is his injectors are going out again. Did GM replace them under the special warranty?

 

If he has a pyrometer, check egts. otherwise, just for fun, disconnect the muffler and hammer on it, see what happens. What its doing sounds like an exhaust system restriction, although poor performance is usually accompanied by high temps.

More info would help, but hey, we'll do what we can.

If it smokes grey a bit, the EGR could be goofing up, had this happen on a few 5500 series GMs with the 6.6

 

The VGT may also be hanging up. EGR problem or boost leak as already described. Plugged fuel filter?