There are two types of loans used by finance companies. One is called the "Rule of 78" and the other is called a "Simple Interest loan".
Rule of 78 Loan
The rule of 78 loan is seldom used when buying an automobile. It is a per-calculated loan with the interest built in each monthly payment.
Simple interest has a daily Perdiem and will change month to month based on how you pay your bill.
Simple Daily Interest Rate Formula