What would make an 08' 6.4 get poor milage even after deletes?

Check the air filter, fuel filters, see if the hubs are locked in. Are there over sized tires on the truck? If so your speedo/odometer will not be accurate.
 
Did the custom tuning disable the intake butterfly valve or is the intake horn replaced with an aftermartket part? I had a guy fron SLC Utah stop here last Saturday with his 05 6.0L...drove it all the way to PA getting 12-13 MPG because the shop that installed the EGR cooler delete kit didn't remove the intake butterfly flap.
 
h&s turns everything off. my 08 averaged 17mpg hand calc'd going from Houston to Dallas and back over thanksgiving.

truck weighs just shy of 10,000lbs and has a service body on it.

running the 250 h&s tune w/ the tranny on S2M
 
The timing on the 6.4 is not very great, it seems to fluctuate, and doesn't always do what you'd think at a point in time.
My guess is that part of the programming is either uncracked, or too difficult for aftermarket devices to modify, because mine tuned is the same.
 
i going to say your map sensor is gotton dirty either by swaping it form teh factory intake or an over oiled air fitler.
 
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