| welcome to the DF...firsf off get rid of that stock exhaust and install a 4" Silverline or MBRP exhaust then do a Programmer from either Diablo,Quadzilla,ATS,PPE or EFIlive and that should gain you roughly 2-4MPG depending on driver..the Diablo is the best for the $$ but the best IMO is the PPE.don't add more then 80-90RWHP on the stock Allison if you want to keep it for the life of the truck.
__________________ James 
TyreBoyz Pulling Team
Pulling Truck called Late Nights.05 LLY 3500 EC LB DRW tuned By Idaho Rob.
Work truck,06 LBZ 2500 CC SB SRW,tuned by Idaho Rob and PPE at times
My Truck now,1987 Chevy 2500 350 tweaked,6" SuperLift,350 V8 bored 60 over,Elderbrock headers Cam and Titanium Rockers,750 Twin Pumped Holley Carb and some other goodies.crank dynos over 400HP. | 
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