I went from getting an average of 17-18 and then it went down to 14.
watching the boost gauge it seems to run higher then it use to
when in park there seems to be a small miss between 1000-1500 rpms
idling theres quite a bit of blue smoke (been like this for awhile) and puffs a little blue when first taking off
had a SCT tuner on for a while but returned it to stock and a couple tanks of fuel later milage dropped it literaly went from 17-18 to 14 in one tank. with the tuner i was getting 20-21.

 

Sounds like a possible bad injector

get it checked for Codes

 

Sounds just like mine it's back at dealership getting 2 more injectors and fuel psi regulator this is the third trip. So far 4 new injectors & fuel pump (HFCM) & the regulator. Thank goodness for warranty, only had to pay deductable 1 time so far. If you know someone with a scan tool that can do the injector contribution test that will let you know whats going on for sure! Good luck!