For purposes of this part:

(a) A heavy-duty gasoline-fueled vehicle is considered to be a complete vehicle if it has the primary load carrying device or container attached at the time the vehicle leaves the control of the manufacturer of the engine, and is considered to be an incomplete vehicle if it does not.

(b) For all other heavy-duty vehicles, a vehicle that has the primary load carrying device or container attached at the time the vehicle is introduced into U.S. commerce is considered to be a complete vehicle. Vehicles not considered to be complete vehicles are incomplete vehicles. For purposes of determining when a vehicle is introduced into U.S. commerce, an assembly of motor vehicle parts is deemed to be a vehicle if either of the following applies:

(1) A piece of equipment that is intended for self-propelled use on highways becomes a vehicle when it includes at least an engine, a transmission, and a frame. (Note: For purposes of this definition, any electrical, mechanical, and/or hydraulic devices attached to engines for the purpose of powering wheels are considered to be transmissions.)

(2) A piece of equipment that is intended for self-propelled use on highways becomes a vehicle when it includes a passenger compartment attached to a frame with axles.