Vehicles acquired by a registrant after the commencement of the registration year and subsequently added to the fleet shall be apportioned pursuant to the provisions of chapter 321 and the international registration plan.
Iowa Code § 326.11
89 Acts, ch 317, §38; 2002 Acts, ch 1063, §41; 2005 Acts, ch 8, § 41; 2012 Acts, ch 1093, § 21