I don't run Bully Dog, but I would have to say that you will get better fuel millage with it on. I believe that it will advance your timming there for make it more efficient. I might now have the correct answer for you but someone will very soon.
I have been reading this site for awhile, figured I better start posting...
My duramax went from about 14 to 18mpg (highway) adding the Bullydog tripledog with no other modifications. I had the same experience with my '04 Ferd powersmoke. It went from about 15 to around 18/19 mpg (hwy). It just seems like the trucks don't work as hard when they are programmed.
You could probably just set the programmer to stock setting and recheck your mileage.
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