Tex. Transportation Code § 451.414: MAXIMUM RATE OF VEHICLE EMISSIONS TAX.

(a) Each year that a board imposes a motor vehicle emissions tax, the board shall set the motor vehicle emissions tax rate as a percentage of the maximum tax rate specified for each class of vehicles in the following table:

(b) The rate of the tax may not exceed 100 percent of the amount specified by the table and applies equally and uniformly to all classes and to all members of each class.

Acts 1995, 74th Leg., ch. 165, Sec. 1, eff. Sept. 1, 1995.