35 ILCS 505/17
It is the purpose of Sections 2 and 13a of this Act to impose a tax upon the privilege of operating each motor vehicle as defined in this Act upon the public highways and the waters of this State, such tax to be based upon the consumption of motor fuel in such motor vehicle, so far as the same may be done, under the Constitution and statutes of the United States, and the Constitution of the State of Illinois. It is the purpose of Section 2a of this Act to impose a tax upon the privilege of importing or receiving in this State fuel for sale or use, such tax to be used to fund the Underground Storage Tank Fund or the Transportation Renewal Fund. If any of the provisions of this Act include transactions which are not taxable or are in any other respect unconstitutional, it is the intent of the General Assembly that, so far as possible, the remaining provisions of the Act be given effect.
35 ILCS 505/17