I am new on this site. I am test driving a 2015 Laramie Crew cab 4x4 ECODIESEL with only 400 miles on it for a few days. It has 3;55 gears. The best mileage I have seen running at 57mph average is 28.2 MPG. Should I expect better MPG as I put more miles on it? Thanks for any comments in advance.