I have an '04 Imperial with the Cummins ISL. I drive from California to Connecticut on an annual basis, a 3200 mile one way trip. I also tow a '99 Jeep Grand Cherokee. Speed and wind resistance play a big role in the mileage you will get. When diesel prices started to go up, I tried slowing down. I now use cruise control set at 55 and the mileage picked up to the 9mpg range. Interestingly, I get better mileage heading east than west. Running west, I usually face headwinds which greatly affect the mileage. The best I ever got was 10.3 heading east and westbound was usually about 1mpg lower. Typically, I can count on the high 8s heading east and low 8s westbound.
Picking up the speed to 65 will usually subtract 1mpg from those numbers. The mileage numbers are obtained from my Aladdin system, but I have verified them over many tankfulls and have confidence in the numbers.
Dick Lucas, '04 HR Imperial.