Whenever you manage to cause imminent boredom even in a crowd of people relatively interested in cars by speaking about them in such detail that people fall off the conversation. I've pulled this off too many times with a particular friend of mine. Oh, also, when the reason you actually got to know one of your best friends is similar taste in cars. Yeah, I'm talking about the same guy.
Finally, when you have such a car nut reputation that it seems logical for your girlfriend to respond to teasing her about her desktop background, featuring a bunch of shirtless Asian guys, like this: "You're the one to talk, damnit, you've got a background with an old BMW in it!"