Mont. Code Ann. § 61-3-724

Motor vehicles, trailers, semitrailers, or pole trailers acquired by the owner after the commencement of the registration period and subsequently added to a proportionally registered fleet must be proportionally registered by applying the mileage percentage used in the original application for the fleet for the registration period to the regular registration fees due with respect to the motor vehicle, trailer, semitrailer, or pole trailer for the remainder of the registration period.