Nothing in this act shall authorize any gross weight of a vehicle, combination of vehicles or axle or axles of a vehicle on the national system of interstate and defense highways greater than permitted by laws of the United States congress, except that this section shall not apply to a truck tractor and dump semitrailer or truck trailer combination authorized under the provisions of subsection (a)(3) of Kan. Stat. Ann. § 8-1909, and amendments thereto.