It varies state to state. Some states have same maximum speed for all vehicles, others slow "trucks" down without defining on signage what is a "truck" and yet others slow down either "trucks" or all vehicles by weight classes. I've seen 26,000 pounds, 20,000 pounds, 14,000 pounds, 10,000 pounds as classification weights. And I haven't seen the whole country, still missing the West Coast and Northeast.
Signage will usually be prominent as you enter the state, signs after that might just say one speed limit then "trucks" and another. The real answers are in the details of 50 different motor vehicle codes.
