Indiana Code Title 6. Taxation § 6-3.5-4-5

Sec. 5. (a) The adopting entity may, subject to the limitations imposed by subsection (b), adopt an ordinance to increase or decrease the surtax rate or amount.  The new surtax rate or amount must be within the range of rates or amounts prescribed by section 2 of this chapter.  A new rate or amount that is established by an ordinance that is adopted after December 31 but on or before September 1 of the following year applies to vehicles registered after December 31 of the year in which the ordinance to change the rate or amount is adopted.  A new rate or amount that is established by an ordinance that is adopted after September 1 but before January 1 of the following year applies to vehicles registered after December 31 of the year following the year in which the ordinance is adopted.  However, in the first year the surtax rate or amount is effective, the surtax rate or amount does not apply to the registration of a vehicle for the registration year that commenced in the calendar year preceding the year the surtax rate or amount is first effective.

(b) The adopting entity may not adopt an ordinance to decrease the surtax rate or amount under this section if: