Mont. Code Ann. § 61-3-317

(1) The new owner of a transferred motor vehicle, trailer, semitrailer, or pole trailer has a grace period of 40 calendar days from the date of purchase to make application for a certificate of title and pay the registration fees, fees in lieu of tax and other fees required by this chapter, and local option taxes, if applicable. However, the motor vehicle, trailer, semitrailer, or pole trailer may not be operated upon the streets and highways of this state during this period unless a temporary registration permit has been issued for and is properly displayed on the motor vehicle, trailer, semitrailer, or pole trailer as permitted by 61-3-224.

(2) If the motor vehicle, trailer, semitrailer, or pole trailer was not purchased from a licensed motor vehicle dealer as provided in this chapter, it is not a violation of this chapter or any other law for the purchaser to operate the motor vehicle, trailer, semitrailer, or pole trailer upon the streets and highways of this state without a current registration receipt or registration decal during the period allowed under 61-3-224 if at all times during that period a temporary registration permit issued under 61-3-224 is properly displayed.

New registration required for transferred motor vehicle -- – grace period -- – penalty -- – display of proof of purchase

Except as otherwise provided herein, the new owner of a transferred motor vehicle shall have a grace period of 20 calendar days from the date of purchase to make application and pay the taxes or fees, or both, provided by part 5 of this chapter, unless the tax or fee has been paid for the year, as if the vehicle were being registered for the first time in that registration year. If the motor vehicle was not purchased from a duly licensed motor vehicle dealer as provided in this chapter, it is not a violation of this chapter or any other law for the purchaser to operate the vehicle upon the streets and highways of this state without a certificate of registration during the 20-day period, provided that at all times during that period a vehicle purchase sticker in a form prescribed and furnished by the department, obtained from the county treasurer or a law enforcement officer as authorized by the department, reciting the date of purchase is clearly displayed in the rear window of the motor vehicle. Registration and license fees collected under 61-3-321 are not required to be paid when a license plate is transferred under this section and 61-3-335. Failure to make application within the time provided herein subjects the purchaser to a penalty of $10. The penalty shall be collected by the county treasurer at the time of registration and shall be in addition to the fees otherwise provided by law.