06 GMC LBZ CCSB. 5" exhaust, EGR blocker, rail shim, TransgoJr, Cognito leveling kit with 295/65/18 Duratracs and a custom EFI Live tune.
07 GMC CCSB LBZ with 4" exhaust, EGR blocker, rail shim, cognito leveling and 275/65/20 Duratracs with edge evo tuner here(stage 3 85hp tune), and airbag system. Almost exact same set up truck.
with 2 sleds at 75 mph round trip from salt lake city to idaho/wyoming I usually get 16.5 mpg. Slow her down to speed limit (65) and im 17.5 mpg. empty runs (with sled deck) I can pull 19-20 at 75mph, 21 at 65mph. These trucks do awesome for fuel when they are set up right!