The national 55 mph speed limit was dropped 15 years ago, so if a state still has one, they either like throwing people in jail (South Carolina) or may have smaller highways that pre-date the Interstate system (mostly the northeast).

For a while after the 1996 change, Montana's speed limit was whatever was "reasonable and prudent" which some people treated like Autobahn speeds. The law was struck down for being vague.

Craziest thing I ever saw: I was flying LHR-DUS and as we approached the runway, we were parallel to a highway I assume was a stretch of the Autobahn. There were cars going faster than the plane, and it was raining!