As a first time diesel owner, I based my knowledge on a close friends 5. 9 automatic. He gets 15. 9 mpg towing his 35ft 5er. My new 6. 7 (only 1500 mi) only gets, 10mpg towing my 30' trailer (7000lbs). I know the mileage improves as they get broken in but I'm just curious what others are getting...