MLeppen said:It is 10,000 # combined truck and trailer, if under 10k ,70 mph is the limit.
That is only partially true.
Here is the blurb straight from the Michigan State MCL website.
257. 627 Speed limitations.
(5) A person driving a passenger vehicle drawing another vehicle or trailer shall not exceed a speed of 55 miles per hour, unless the vehicle or trailer has 2 wheels or less and does not exceed the combined weight of 750 pounds for the vehicle or trailer and load, or a trailer coach of not more than 26 feet in length with brakes on each wheel and attached to the passenger vehicle with an equalizing or stabilizing coupling unit.
(6) A truck with a gross weight of 10,000 pounds or more, a truck-tractor with a trailer, or a combination of these vehicles shall not exceed a speed of 55 miles per hour on highways, streets, or freeways and shall not exceed a speed of 35 miles per hour during the period when reduced loadings are being enforced in accordance with this chapter.
According to the state website, my 2-car gooseneck is considered a semi-trailer as well.

257. 59 “Semi-trailer” defined.
Sec. 59.
“Semi-trailer” means every vehicle with or without motive power, other than a pole-trailer, designed for carrying persons or property and for being drawn by a motor vehicle and so constructed that some part of its weight and that of its load rests upon or is carried by another vehicle.