New tires of proper size and ply rating commensurate with the chassis manufacturer's gross vehicle weight rating shall be provided. All tires on any given vehicle shall be the same size and ply rating. Dual rear wheels and tires shall be provided on all vehicles. If the vehicle is equipped with a spare tire and rim, they shall be of the same size as those mounted on the vehicle, and if the tire carrier is specified, it shall be mounted in an accessible location outside of the passenger compartment.