Question Unless otherwise posted, the speed limit on interstates is: Answers 70 mph. 55 mph. 45 mph. 35 mph. Explanation Unless otherwise posted, the speed limit on interstates is 70 mph. If a different speed limit is posted, you should follow that speed limit. Pass your DMV Written Test with flying colors Take me to the cheat sheet North Carolina This questions appears in the following tests: North Carolina Practice Test 9 North Carolina Practice Test 37