High boost, Bad mpg's

[06 Strokin]

Alrite guys, so on the flatest ground I can find around here cruising at 70 my boost is around 8-9psi. The slightest incline will shoot it up past 10psi. Im getting bad mpgs as well. I just filled up after going around 173 miles (city/highway) I averaged 11mpg. Im running the SPD tune right now. It just seems like it takes a lot to keep it at 70. What do yall think the problem could be? I've tried different tunes and none improve it. Im out of ideas. The truck has 103,xxx miles on it. Just put 35" mud grapplers on it about a month ago, but it was doing this as well when I had 33's.

[Int.Tech04]

cruising at 70mph with 35X12.50's, im around 9psi on flat ground, but climbs accordingly depending on the incline. never full boost, just a few PSI higher. i dont remember the exact PSI though. ive also noticed worse MPG's since adding the tow tune to it from my SCT livewire TS. sounds like its custom tune time.