Mass. Gen. Laws ch. 255B § 14
A retail seller may charge, receive and collect for any new or used motor vehicle, a finance charge not in excess of an annual percentage rate of twenty-one per cent.
Such finance charge shall be computed on the amount financed as determined under chapter one hundred and forty D on contracts payable in successive monthly instalments substantially equal in amount.
On contracts providing for instalments extending for a period less than or greater than one year, the finance charge shall be computed proportionately. The finance charge may be computed on the basis of a full month for any fractional month period in excess of fifteen days.
When a retail instalment contract provides for unequal or irregular instalments, the finance charge shall be no more than the effective rate provided in this section, having due regard for the schedule of instalments.
The finance charge shall be inclusive of all charges incident to investigating and making the contract, and for the extension of the credit provided for in the contract and no fee, expense or other charge whatsoever shall be taken, received, reserved or contracted for except as provided in this section and in section eleven and section seventeen and for those items expressly provided for in the retail instalment contract as set forth in chapter one hundred and forty D.
Mass. Gen. Laws ch. 255B, § 14