When you finance through the dealership, you have to remember that the Finance and Insurance (F&I) department is often a bigger profit center than the sales department. The business manager (the person you deal with in the F&I department) sends your credit information to the lender(s) they deal with. The business manager then takes the lowest approved interest rate and marks it up (increases it). The marked-up amount is the dealership's profit on the financing. There is no law saying the dealer has to reveal that mark-up to you. This is why you have to keep your negotiating hat on throughout the process! This financing is really just another product the dealership sells, known as a Retail Installment Sales Contract (RISC).