Is money acquired by a son from selling a car gifted to him by his father, without the father's knowledge and consent, considered unlawful money?

If your father transferred ownership of the car to you before selling it, then you are permitted to dispose of it through sale, gift, or similar means, provided you are a mature and رشيد (of sound judgment). However, if it was merely a loan or a usufruct without transferring ownership, then you are not permitted to dispose of it. Your sale of it would be considered an unauthorized disposition (تصرف فضولي) that would not be effective unless approved by your father. The mere registration of the car in your name does not indicate ownership unless there are supporting contextual clues (قرائن).