You say 15" and then you ask what size wheel, so I assume yu mean width? 15X7 will comfortably fit any tyre you want to put on a Z. If you mean what size tyre, then you have fallen into the most often asked question catagory. Lowered? Here's the thing: Compare 35 different pix of Z's (lowered, stock, low profile, tall, 0 offset, +1 offset, coilovers, etc, etc, etc, etc,) and you will see 35 different tyre/fender relationships. Springs sag, strut isolators are different heights, all kinds of thigs happen to make things variable. You will find tons of tyre recommendations. I have had 195/75/14's 195/50/15's unlowered and 195/50/15 and 225/50 15's lowered. They all work fine. Some have more gap than you probably want. just do a bunch of research.