Mich. Comp. Laws § 257.77: “Truck tractor” defined.

Sec. 77.

"Truck tractor" means every motor vehicle designed and used primarily for drawing other vehicles, and not so constructed as to carry a load other than a part of the weight of the vehicle and load so drawn, except that a truck tractor and semitrailer engaged in the transportation of automobiles may transport motor vehicles on part of the power unit.