Here's the verbage on my contract, which I read BEFORE I signed for the car:
15. Prepayment. I may prepay the unpaid balance of the Amount Financed in full or in part at anytime without penality. If I do so, I must pay the earned and unpaid part of the Finance Charge and all other amounts due upon the date of payment.
In other places it describes the Finanace Charge as interest incurred, which may be more or less depending on early or late payments, etc... There are no other 'amounts due' except the principle on this contract. You simply owe the interest (Finance Charge) for the period the loan was outstanding plus the unpaid principle.
The paper work says there is no penalty to me for paying off early. We told the loan dude we will pay in full the first installment, he didn't even blink. So evidently no issues on their end either.
One last point: My Contract says "Oregon Bankers Assoc." on it and "Simple Interest - Oregon". Your state may be completely different.
I would suggest going into the dealer, getting a copy of the contract, bring it home and read it. My dealer gave me a copy to read while they were busy figuring. I wrote all over it so they didn't ask for it back. The salesguy isn't the finance guy, I wouldn't trust either.