Mich. Comp. Laws § 445.1307

Sec. 307. Subject to the provisions of section 301, the finance charge to be included in a consolidated time balance shall be determined by applying a finance charge at a rate not exceeding the maximum rate specified in that section to either: (a) The total of the principal amount financed under the subsequent sale and the unpaid balance of any previous contract determined by deducting from the unpaid time balance thereof as of the date the finance charge is to accrue on the subsequent sale, any then unearned finance charge in an amount not less than the refund credit provided for in section 303 computed without the allowance of any minimum earned finance charge, for the period from the date the finance charge is to accrue on the subsequent sale to and including the date when the final installment of such consolidated time balance is payable; or (b) The principal amount financed under the subsequent sale for the period from the date the finance charge is to accrue thereon to and including the date when the final installment of such consolidated time balance is payable and, if the due date of the final installment of such consolidated time balance is later than the due date of the final installment of any previous contract included in the consolidated time balance, on the unpaid time balance of such previous contract as of the date the finance charge is to accrue on the subsequent sale for the period from the date when the final installment on the previous contract would have been payable to the date when the final installment of such consolidated time balance is payable.