Speed limits are not posted on state-designated ORV trails, but you may not operate an ORV at a rate of speed greater than is reasonable and proper based on existing conditions.
Additionally, ORVs may not be operated in a careless manner, in any attempt to kill any animal or bird, while transporting uncased bows or loaded firearms (with exceptions), while transporting open alcoholic beverages and other considerations.
For more information refer to the Handbook of Michigan Off-Road Vehicle Laws that outlines laws governing off-roading and safe ORV operation.