Trailer tires are rated for 65, so I try to stay under. The problem is the truck is in the sweet spot between 65 and 70. If it's flat or down hill I'm usually 60-65. Uphill, I push it up to 67-68 to stay in the power band. If I let it get below 60 on any incline, it's downshifting or lugging.
2014 Ram 2500 Mega Cab Laramie 4x4 6.4L HEMI 4.10s with antispin
2014 Jayco Jay Flight Swift 287BHBE
Equal-i-zer 90-00-1200(new 90-02-4900 shank)
2x Honda 2K