The original amount financed with your auto loan, not to be confused with the remaining balance or principal balance. Your proposed extra payment per month. This payment will be used to reduce your principal balance. Monthly principal and interest payment based on your original loan amount, Free and easy-to-use automated calculator which quickly estimates your monthly car loan payments & helps you figure out how expensive of a car you can afford to buy given a set monthly budget. Create A Loan Amortization Schedule.