2005 chevy 140000km the tail pipeis really black and it is hard on fuel. getting about 15mpg.There are no codes. It's a 4.8 4by4 ex cab auto. A friend of mine has the same truck an gets 20 all the time on the highway .Truck runs fine and has lots of power. When i pull my 25ft 5th it drops to 10 mpgany ideas.