Mont. Code Ann. § 61-3-720

Sections 61-3-711 through 61-3-733 relating to proportional registration of fleet motor vehicles, trailers, semitrailers, or pole trailers may not be construed as requiring any motor vehicle, trailer, semitrailer, or pole trailer to be proportionally registered if it is otherwise registered in this state for the operation in which it is engaged, including but not limited to regular registration, temporary registration, or trip permit or registration.