"Farm tractor" means every motor vehicle designed and used primarily as a farm implement for drawing plows, mowing machines, and other implements of husbandry.
"Road tractor" means every motor vehicle designed and used for drawing other vehicles and not so constructed as to carry any load thereon either independently or any part of the weight of a vehicle or load so drawn.
"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.