I don't. Because when it breaks down and I have to leave it on the side of the road, I don't want to feel like I'm abandoning a friend. So, no name for my cars. Instead, I refer to them by their brand name. "Go warm of the Lexus", "Go put gas in the Chevy", or "When do we have to take the Audi in for an oil change?" Nice, and impersonal.