58mph for me. I feel good like I'm making up time. My mileage is much better at 58 as compared to 65. I'm going slow enough so that when you pass me doing 65 or 75 pulling that trailer swaying behind you; you are passing me quickly and I don't have worry as much about that trailer you are pulling swaying across the lines and almost hitting me.
And yeah ST tires - have you ever picked up an unmounted one. Feel that "thickness-less" of the sidewall. Notice the lack of deep tread. I run "D" range ST tires on my 6,000 GVWR trailer, I run LT on my 14,400 GVWR (dual 6,000lb axles, the rest is on the pin) 5th wheel.