Replacing tires: IMO, it doesn't make sense until they need replaced for other reasons (age or wear). Why waste perfectly good tires?
I tow at 60. When I first got the trailer, I did some experiments without the family with me and in good weather just to see what was actually possible and comfortable. Getting up to higher speeds is easy, but I found that the setup started to feel uncomfortable in the neighborhood of 70-75. (At the time, I was quite pleased with this as the PUP that preceded the trailer always felt uncomfortable even at 60 - it's amazing the difference a good hitch makes.)
