My truck and trailer tires are rated to 130 mph but I try to keep it a little below that :) The truck gets its best mileage at 50.
However, the greater disparity between your speed and the speed of vehicles in your travel lane the more likely you are to have or cause an accident--SO I drive with the flow of traffic. In west TX that is 75-80 in the slow lane, in CA it is 55-60 in the slow lane.
BTW: My truck and trailer combo stops quicker than the truck alone--8 brakes have more friction than 4, especially when the back tires are unloaded ;)