I just called the iPass Customer Call center and they said you HAVE to have a transponder in every car you own. You can use the same transponder in all your cars, but you have to have it when you go through tolls. I just received my Model X and after seeing this post was pleasantly surprised thinking I didn't have to attach a transponder, however finding it difficult to reconcile with what the iPass Call Center is telling me. How are you sure you don't need a transponder in every car? Just trying to understand. Thanks.