I have a levelling kit with bigger tires (~34" compared to the ~32" stockers), which caused the biggest drop. I also have a replacement muffler, which doesn't really affect mileage, but does affect how often I get into it. It sounds GREAT! I really am pretty happy with 16-17 mpg out of a fullsize truck with 390 hp and 407 ft-lbs of torque. 
