Gross combination weight rating means the greater of (1) a value specified by the manufacturer of the power unit, if such value is displayed on the Federal Motor Vehicle Safety Standard certification label required by the National Highway Traffic Safety Administration, or (2) the sum of the gross vehicle weight ratings or the gross vehicle weights of the power unit and the towed unit or units, or any combination thereof, that produces the highest value. Gross combination weight rating does not apply to a commercial motor vehicle if the power unit is not towing another vehicle.