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If the obligation underlying the secured claim was used to buy the collateral (e.g., a car loan), the plan must provide for full payment of the debt, not just the value of the collateral. Here is a link to the US Bankruptcy site that discusses this fact: http://www.uscourts.gov/FederalCourts/Bankruptcy/BankruptcyBasics/Chapter13.aspx
Therefore, you'd have to pay the full amount for the car - principle and interest in order to keep it.