6500 isn't bad. I have pulled my GVWR 7525 camper behind my 2002 Ranger with 3.0 and trailer brakes on both camper axles. I would not recommend it for those less skilled. What does your payload sticker read if you don't mind me asking ?
Most of the interstate system is 70. Most of the state highways are 55. Makes sense to test at 55. Most people have a daily commute consisting of state and county roads/hwys where they might not go faster than 55.