Michigan specifies a tire width weight restriction of 700 lbs per inch of tire width. This applies to super single tires as well. As a general rule, your super single tires should be at least 13″ (330mm) to take advantage of the same axle weight restrictions available for dual tired axles under Michigan law.
The Big Truck Guide database now has the super single tire weights explicitly listed for Michigan Law combinations. More information can be found on the PDF document called ‘Maximum Axle Loadings in Relation to Tire Size’ found on the following page: