625 ILCS 5/3-402.1
Any owner or rental owner engaged in operating a fleet of apportionable vehicles in this State and one or more other states may, in lieu of registration of such vehicles under the general provisions of Sections 3-402, 3-815, 3-815.1, and 3-819, register and license such fleet for operations in this State by filing an application statement, signed under penalties of perjury, with the Secretary of State which shall be in such form and contain such information as the Secretary of State shall require, declaring the total mileage operated in all states by such fleet, the total mileage operated in this state by such fleet during the preceding year, and describing and identifying each apportionable vehicle to be operated in this State during the ensuing year. If mileage data is not available for the preceding year, the Secretary of State may accept the latest 12-month period available. "Preceding year" means the period of 12 consecutive months immediately prior to July 1st of the year immediately preceding the registration or license year for which proportional registration is sought.
Such owner shall determine the proportion of in-state miles to total fleet miles. Such percentage figure shall be such owner's apportionment factor. In determining the total fee payment, such owner shall first compute the license fee or fees for each vehicle within the fleet which would otherwise be required, and then multiply the said amount by the Illinois apportionment factor adding the fees for each vehicle to arrive at a total amount for the fleet. Apportionable trailers and semitrailers will be registered in accordance with the provisions of Section 3-813 of this Code.
Upon receipt of the appropriate fees from such owner as computed under the provisions of this Section, the Secretary of State shall, when this State is the base jurisdiction, issue to such owner number plates or other distinctive tags or such evidence of registration as the Secretary of State shall deem appropriate to identify each vehicle in the fleet as a part of a proportionally registered interstate fleet.
Vehicles registered under the provision of this Section shall be considered fully licensed and properly registered in Illinois for any type of movement or operation. The proportional registration and licensing provisions of this Section shall apply to vehicles added to fleets and operated in this State during the registration year, applying the same apportionment factor to such fees as would be payable for the remainder of the registration year.
Apportionment factors for apportionable vehicles not operated in this State during the preceding year shall be determined by the Secretary of State on the basis of a full statement of the proposed methods of operation and in conformity with an estimated mileage chart as calculated by the Secretary of State. An established fleet adding states at the time of renewal shall estimate mileage for the added states in conformity with a mileage chart developed by the Secretary of State.
625 ILCS 5/3-402.1