I'm feeling like not giving back the car my dad let me borrow.
Was he expecting it back soon?
No, I just thought I should hang onto it for awhile.
Your dad let you have his car in good faith that you would respect him and his car. It's not acceptable to refuse to return something that was borrowed. Talk to your father about using it for an extended amount of time first or you may not be able to use it again next time.
Bullshit I can do what I want with my own stuff.
But the thing is it's not your car, so it's not your own stuff. I would highly recommend you return the car to him at a reasonable time, if you don't he most likely it will not let you borrow it again.