Just a couple things here.  My truck is an 04  LLY.  I bought it from the lower mainland at sea level.  I live approximately 600 meters, or 1969 ft above sea level.  now my question is, will this affect fuel mileage, would my truck be programmed for lower elevations?  Could the fuel/air mixture be different between the 2.  
I'm only getting around 12 MPG and the GM dealership doesn't know why? They told me give em $300.00 and they will run some tests, but come on now.  
Please, some advice would be sooooo helpful.
thanks  

12 unloaded? Yeah, that's pretty bad. I guess the first thing I'd do is check the usual suspects like tire pressure, condition of the trans and diff. fluids. My hunch is it could be a bad sensor which is sending a faulty signal and over-fueling the engine. Have you noticed much visible smoke?  

I am getting the torque converter changed this week (warrenty), do you think that may be a factor?  All the fluids are topped up and i changed them all when i first got it in Nov.  The check engine light is NOT on so i never thought of a bad sensor.  Would the engine light be on if it was a bad sensor?  

if the truck is under warranty than they are responsiable for checking why it is getting such poor mileage not you giving them 300 dollars to check it out if they dont want to work with you than call gm hot line and ask why they want you to pay when they are responailbe for any problems you have not what they want to cover there just trying to take advantage of you a loyal customer or just try to complain to the manger of the way they are trying to treat you  

it might be a common thing among drivers is not keeping your foot out of it, or possibly you can always check your air filter, tire psi., etc., the small things robs fuel mileage..