The answer has more to do with cultural than language. "Could you return my car to me" could be rude or it could be perfectly acceptable, it just depends on the cultural context. Language-wise it's fine. A softer way to say it might be something like, "When you get a chance, could you return my car to me." This is more of a reminder than a demand. It depends on how forceful you want to be.

Are you finished with my car? I really need it back. As someone else said, cultural norms dictate these sorts of exchanges.

Another way If you are really worried how they other person might react why not just add the word 'grateful' to your sentence like this : 'I'd be grateful if you could return the car back please'

What is rude is his not returning the car. It is YOUR car, after all. Just say that you need it by a certain date and ask him if he could please return it by then. Or ask him if his need for it has passed. If so, ask him to please return it.

The second answer was very good. I just wanted to point out to you that with borrow you need to say borrowed something from me. Just in case, explain to me also. A lot of my students say , "Can you explain me but it is always to me unless you are just saying can you explain why ......... With lend you can say , "He lent me (something) but borrow with me , always needs from.