We donated a Dodge Caravan to charity, and I believe they attached the KBB (Kelly Blue Book) value, adjusted for mileage, options. etc.
One thing to be sure to do. Check to see if you are actually donating the vehicle to the charity directly, or if there is a broker involved. If there is a broker, try to find out how much of the proceeds actually get to the charity. IIRC, there may be some tax issues with claiming the full retail value as a donation when the charity actually ends up with far less due to broker "fees".