The mandatory 55 mph limit imposed by the federal government of the US ended in late 1995.
While some stretches of road, especially in the western plains, will allow for speeds in excess of 90 mph, you won't find many State Highway Patrol officers that will allow you to travel appreciably above the highest posted rate of 75 m.p.h.
The most typical highway posted rate has been 65m.p.h. since the 55 m.p.h rate mandation was repealed.