Originally Posted by CDepp

Get a Power of Attorney for your dad specifically for the car. Then, you can sign his name when you decide to sell. Otherwise, any dealer/lender (and, if he's on the title - the state) will require his signature to release the lien for payoff (and the title transfer if his name is on it).

Edit for clarification:
If a company (dealer or lender) is going to pay off your loan, and you and your father are both on the loan, they will require both his and your signature to sign what's called a 10-day Per Diem Payoff Letter (generally - generic term). It's a document that states how much your lending company will accept to clear the amount owed on the loan.

From the payoff, it takes an average company (BB&T, Wells-F, etc) 10-15 business days to clear the payment - even if done via wire transfer or cashier's check. At that point, the release of title' is performed. If buying from a dealer, at this point, they will take your keys, have you sign a Bill of Sale, and you're free of the car. If buying from an individual, and they don't have a title in hand, you could wait another 20-30 days for the release and title transfer in the following info before you have a title in your hands).

Depending on your state, the title can be held by your bank or by the state. If the release of title is done by the state, then there are two options, written or electronic releases. The release itself once received only takes a day or two, it's the method (elect or paper) that will add more days to the process. Depending on the buying company, they may have the title mailed to you or themselves from the DMV as their check will clear the old loan, not a check from you.

If your father's name is also on the title, he will need to sign it with you (at the bottom) to sell the vehicle.

So, all in all, getting your dad to sign a PoA may cost $45-$125 for the document fees, or the new lender may have you both sign them for the company to forge your names, but at some point, both you and your dad are legally bound to the loan. If his name is on the title, then he is also legally bound to the state registration for the vehicle. The PoA will allow you to sign as him for release of title, loan payoff, and (if applicable) the title transfer.