damn...i dunno what full size you guys are running...but my pops has a 03 z71 w/ the 5300 series engine (5.3L V8) and he has flows with k and n intake and a hypertech tuner on the 87 tune and he averages 13.4. It has that little thing in the guage cluster. He doesnt drive it hard at all. About 40 percent hwy driving. He's rolling stock tires right now...he used to have 32's..they wore out and i changed him back to stockers for the gas milage and a few other reasons.
that milage is with the STOCK tires..with the 32's and the hypertech speedo fix it was getting like 14.1
i dunno whats up with it. it's got 20k miles on it, been fully serviced already, oil changed, tires filled..everything. I work on his truck all the time for him
and also I have a friend with a suburban w/ the same engine that gets 13.2 average. He drives about 60 percent hwy. Kind of strange...