Unless otherwise posted, the speed limit on state highways is: Question Unless otherwise posted, the speed limit on state highways is: Answers 25 mph. 35 mph. 45 mph. 55 mph. Explanation Unless otherwise posted, the speed limit on state highways is 55 mph. If a different speed limit is posted, you should follow that speed limit. This questions appears in the following tests: Michigan Practice Test 3 Michigan Practice Test 30