I feel kinda dumb asking this. I have an '88 f250 centurion dually with a 7.3 5speed and gear vendors. It'll be hauling a 35 ft 5th wheel. Dont know the weight of trailer yet. Gvwr on trailer is 13,6xx. I wont run it in double overdrive while towing. What is a typical hauling speed? I was figuring 65-70. Ill be taking it south where the speed limit is 80 for about 3 hrs of the drive.