Tex. Fin. Code § 353.106

A retail installment contract that is payable other than in substantially equal successive monthly installments or the first installment of which is not payable one month from the date of the contract may provide for a time price differential that does not exceed an amount that, having due regard for the schedule of payments, provides the same effective return as if the contract were payable in substantially equal successive monthly installments beginning one month from the date of the contract.