For purposes of Section 3289, the rate of the contracted finance charge shall be the legal rate of interest stipulated by a retail installment contract subject to Chapter 1 (commencing with Section 1801) of Title 2 of Part 4 of Division 3.
Source:
Section 3289.5, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CIV§ionNum=3289.5..