My 2010 Sierra 4X4 with the 6.2 and 3.42 gears was getting 18 to 19 mpg on the highway at 75 mph. I installed a 4 inch Rancho suspension lift (no other changes)in order to have more fun offroading. My mpg has dropped to 15 to 16. I guess getting the vehicle lower to the ground helps the mpg. Time to put on some good headers and a retune for some more mpg and HP.