I have a 2006 GMC Sierra 2500HD with a 6.0, is there a chip I can buy to improve fuel mileage or can I change the rear end?

 

I have the same engine and here is what I done to get better mileage, put in a k&n air cleaner, synthetic oil, filled tank and put it in the garage, closed garage door and now I can get a whole 2 weeks out of a full tank. ethanol sucks so just skip the filter and use the garage door advise.