15-17 on average here with SCT by DJ Looney tune. I have seen 18 a couple times. Doesnt really seem to change no matter how I drive. I filled up and drove 150 miles to a show last year, left in a huge cloud of black smoke, drove the 150miles back home. Got up the next morning, drove 45miles to the dragstrip, ran 3 times, drove bout 30 miles and refilled the truck, still got 17mpg.
Im sure I could switch to a less "hot" tune like the street file(which is supposed to be a great highway tune) and get alil better mileage. But I honestly have no desire to sacrifice power for mileage, in an 8300lb truck. I do enough power sacrificing during the week with my 4cyl Ranger that gets 30mpg.