Why not put the difference in your pocket and not the banker's pocket. Many times, a dealer's bottom line net profit is made in the finance office. It's perfectly legal and has been going on for years. Fortunately, many states have limited dealers in the amount that they can mark up a loan. Informed consumers are now shopping the car loan before signing on the dotted line.
Banks do the same thing.
It's the old maxim of charging whatever the market will bear. If you don't believe me, call the dealer and ask what interest rate they charge. Better yet, call your banker. You won't get a straight answer. The Better Way There is a way to beat the banker.