Depends on the state. Some states have lower, or higher speed limits within their borders. For example, I-10 West of San Antonio, in Texas, has a speed limit of 80mph (by necessity, I'll say!), and State 130 between Austin and San Antonio has a speed limit of 85mph. Utah has some highway sections at 80mph, and most states in the central-west region of the US have highway limit at or around 75mph.
Last time I was in the states, they had rather slow limits on the Freeways. Has this improved yet?