• Interstates: 55-65 metro, 75 plains, 50-75 mountains
• U.S Highways:  55-65

A word to the wise. You may be able to safely take mountain corners marked 45 MPH at 20 over, but those marked 10 or 15 will a lot safer at 0 to 5 over. The tight switchbacks are often bumpy or littered with gravel and have been known to land a few folks on their heads.