I must say, I really admire your ability to honour your gut feelings, most people are in the habit of ignoring their inner warning system, and then you see them up on Yahoo Answers whinning about things going wrong in their lives.
Nevertheless, having already sensed that something is "off" about this guy, you shouldn't have allowed yourself to become 'buds' with him at work. So, unless you REALLY need his support in your work, it might be wise to begin to distance yourself from him in the office. Limit your conversations to a simple hello when you pass by him and, hopefully, your office closeness will gradually begin to fade. As for your car, you could try using your "principles" as an excuse - because no one can argue with a person's principles. For example, you could say: "I really wish I could, but my personal principle is not to fraternize with co-workers outside work. I hope you understand?".