"Truck tractor" means the noncargo-carrying power unit that operates in combination with a semitrailer or trailer, except that a truck tractor and semitrailer engaged in the transportation of automobiles may transport motor vehicles on part of the power unit [...]

(72) "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.