
Quoting
Mr. Knowitall
If you let the car be repossessed or try to turn it in, the lender will normally auction the car (resulting in the lowest possible sales price), then collect from you the difference between the sales price and what you owe (probably with additional charges for the repossession and the cost of the sale). They will demand a 100% payoff, not a continuation of your payments. Either way, repo or turning it in, you will get a significant negative entry on your credit.
If you really can't afford the car, your best approach is normally to sell it for the most you can get, then pay off the loan, making up the difference out-of-pocket.