Whatever the sign says. If you go even 1 mph over, you can get a ticket. If there's a foot of snow on the ground and it's unsafe to drive the speed limit, you can get one for failing to use due caution. The highest a speed limit can be on any road in the U.S. is 70mph. Most cops will not pull you over if your going 5 over the speed limit, usually you can go 10. The speed limit is 70 in Michigan but the flow of traffic is usually about 75, I've never known anybody to get pulled over for 77 or less..........