damn idk why all your 5.9's are doing so horrible, my 5.2 is getting an average of 15.9 in the winter time right now. my brothers 5.9 is getting around 12 and his has a bad plenum gasket. do you guys think it might be time for a full tuneup?
The best thing i think to do if your getting crappy mileage i like to do this, i take off every single wire and inspect them for splits or cracking. i take every single plug out and check to see if there bad if you dont think they are clean them, take your cap and rotor of and inspect those for corrosion and clean the points. this allways helps, get into that throttle body if its dirty! that was allways one of the best things to help out mpg and throttle response, get in there and clean that thing spotless. and of course inspect your hoses and lines for leaks and everything. also the traditional fuel injector cleaner in your gas tank mixed with a bit of mid-grade or premium helps too.