There's no particular risk, because you can't claim any deduction over $500 for a vehicle unless the charity gives you a signed form 1098-C. This form will indicate the price the charity received for the vehicle (assuming they sold it) or it will indicate that they are using the vehicle as a vehicle for their charitable purpose.
Regardless of what you think the value of the car is, the amount you can deduct is the amount the charity got for it when they sold it. Only if the charity keeps the car and uses it as a vehicle, can you deduct the fair market value, which you would need to determine with one of the blue books or auction records or something similar.
Without the 1098-C, you could claim a value of $500, or you wait until you get the 1098-C and file an amended return when you get the form back from the charity. They have up to 3 years to dispose of the car and give you the 1098-C.
