If you haven’t secured your own financing before going to the dealership than you are relying on the Finance salesperson at the dealership to arrange the financing and get the best rate for you. However, they may shop for the most profitable loan for the dealership and not for you. The most common dealer trick is to get you approved at the lowest rate and later add a couple extra points on it. For example, they may get you 6% interest rate but tell you the best rate they were able to find is 9%. Remember, that financing is the good way for the dealer to make more profit.
You may be able to avoid this dealer fraud if you know your credit score. Also get information about the current interest rates to know what interest rate is right for you. If you think the interest rate the dealer offers is too high tell him he needs to do better. If the dealer tells he can’t get you a better rate, tell him you will arrange your own financing. Probably your dealer will calculate the numbers again and you will win. However, make sure that the dealer did not increase the length of the loan to keep your payment lower.