I am asking for a guy i work with. He just traded his 2013 sierra 1500 for a 2013 sierra 2500 with the 6.0. The truck now has 700 miles on it and he is worried cause he is only getting 10-12 mpg. While others say they are getting 15-16 but also have higher mileage. At what point does the mileage improve. He is 62 years old and drives like it also.