Split speed limits (80 mph cars/70 mph trucks) occur in the majority of states. 99% of the states have the right lane law for trucks; the meaning is that must travel in the right lane, but may use the left lane to pass.
Regardless of whether or not states allow an increased speed limit for trucks, chances are most won't be taking advantage of it. Most trucking companies try their best to keep costs down, as most thriving businesses do, and in doing so they "govern" their to usually no more than 65-68 mph to save on fuel costs.
As a former CDL holder and the wife of a professional truck driver, increasing the speed is also considerably more dangerous.
How often behind the wheel of an auto have you had to slow down at a moment's notice and/or swerve to avoid a potential accident? Your average 40 ton semi faces inertia when they try to "swerve" at a higher rate of speed and increase the chance of jack knifing their rig. Domino effect would be semi blocking the lane and no one going more than 35 mph while waiting on emergency services to clear the accident, which can take hours.
Not to mention that type of accident could, worse over, take a life