What is the tire size of the spare? Has anyone just taken it out to gain more storage space or is it possible that the loss of weight in the rear would affect the balance of the car? Does anyone think it's a bad idea to get rid of my current spare which is a 17" wheel (my car has 19" wheels) and put in the emergency spare instead?

If the bottom tray is the one used on the 'is models, what is the tray above it for and in what cars is it used?