Ind. Code § 6-3.5-5-7

Sec. 7. (a) The adopting entity may, subject to the limitations imposed by subsection (b), adopt an ordinance to increase or decrease the wheel tax rates. The new wheel tax rates must be within the range of rates prescribed by section 2 of this chapter. New rates that are established by an ordinance that is adopted after December 31 but on or before September 1 of the following year apply to vehicles registered after December 31 of the year in which the ordinance to change the rates is adopted. New rates that are established by an ordinance that is adopted after September 1 but before January 1 of the following year apply to motor vehicles registered after December 31 of the year following the year in which the ordinance is adopted. However, in the first year the wheel tax rates are effective, the wheel tax rates do not apply to the registration of a vehicle for the registration year that commenced in the calendar year preceding the year the wheel tax rates are first effective.

(b) The adopting entity may not adopt an ordinance to decrease the wheel tax rates under this section if: