Wis. Stat. § 78.405: Annual adjustment of the tax rate.

(1) Before April 1, the department annually shall calculate the rate for the tax under s. 78.40 as follows:

(a) Determine the standard number of British thermal units per gallon generated by gasoline.

(b) Determine the standard number of British thermal units per gallon generated by each kind of alternate fuel that is sold in this state.

(c) For each kind of alternate fuel sold in this state, divide the result under par. (b) by the result under par. (a).

(d) For each kind of alternate fuel sold in this state, multiply the result under par. (c), expressed as a decimal, by the rate for the tax under s. 78.01 as adjusted for the current year under s. 78.015.

(2) The rates determined under sub. (1) are effective on the April 1 after they are calculated.