What I'd do is amortize 3.9% for 72 months and take a look at the interest/principal ratio and see if there is any pre-payment penalty, if any. In as much as the prime is so low, you might be better off securing a loan from your credit union (so long as your base credit score is good) and paying the interest to them (that's where they get the money to pay salaries, etc), rather than paying the salary of some yahoo in Reno, Nevada.
Under Rule 53S of the Truth-In-Lending Law, there can be no penalty for early payment of loans secured through public banks and private credit unions, but there can be a pre-payment clause in privately secured loans from other lenders.
You might want to give serious thought to keeping the funds local.
The other thing that goes on with that type of arrangement is that the dealer gets an incentive that you never see. He or she gets a financial incentive to 'sell' you the packaged financing. It may be a cash payout or a credit, but it's there in one form or another. That's not all bad, however, you might be able to negotiate a lower rate locally. Remember the Overnight Lending Rate (Prime) is at a historic low and people, in general, aren't borrowing. If you are, you are in the drivers seat so to speak. Take full advantage of that.