I find 63 is the most fuel efficient speed. I never had a work vehicle with cruse control, so I have a habit of driving without it. I am not comfortable towing with the cruse control and I would NEVER use it on wet roads.
With all that said, I find that my speed does vary around 65 to 70 if there is no one around. I do try to keep it around 63.
Jimm Zajicek (Zi/)2016 F250 power stroke diesel / 2015 29.9 RE Wildwood Heritage Glenn TT