Q: What wheel sizes and offsets do I need?
A: Generally speaking, there are a LOT of different combinations that people are running on their Z or G.

Let's get the basics out of the way first: You need a lug pattern of 5x114.3, width between 8 and 11 inches, and diameter of 17", 18", 19", or 20".

Here are some of the more popular wheel and tire sizes (for good reason) that people put on their Z's. There is no "RIGHT" answer but there is a limit to how big or small you can go.

18" combination:
Front 18" x 8.5" , Offset +15 to +25, Tires 245/40/18
Rear 18" x 9.5" , Offset +15 to +25, Tires 275/40/18
(lower offset such as +10 in rear is possible but may require rolling)

Front 18" x 9.5" , Offset +25 to +30, Tires 255/40/18
Rear 18" x 10.5" , Offset +28 to +35, Tires 285/40/18 or 295/35/18
(lower offset such as +23 in the rear is possible but may require rolling)

19" combination:
Front 19" x 8.5" , Offset +15 to +25, Tires 245/35/19
Rear 19" x 9.5" , Offset +15 to +25, Tires 275/35/19
(lower offset such as +10 in rear is possible but may require rolling)

Front 19" x 9.5" , Offset +25 to +30, Tires 255/35/19 or 265/35/19
Rear 19" x 10.5" , Offset +23 to +32, Tires 285/35/19 or 295/35/19
(lower offset such as +23 or wider tires in the rear is possible but may require rolling)

From this, you get the idea of what is required to go with smaller wheels like 17's, (typically lighter, faster acceleration), or bigger wheels like 20's (bigger, heavier, usually makes you feel slower).

Get to know how to use the following tools and you will be able to figure out for yourself the correct offsets and tire sizes you need:
Tire size calculator (click)
Wheel Offset Calculator

Certain combinations of LOW OFFSET and/or WIDE WHEELS will require what is known as "fender rolling" or "fender cutting".

If you are not familiar with the term "fender rolling", here is a quick but useful lesson you can learn in a couple of minutes: Go outside to wherever your Z is parked. Run your fingers along the inside edge of a REAR fender just above your tires. You will feel a sharp edge and a flap of metal sticking inward towards the car. Now, go up to the FRONT fender, and feel the inside of the fender again. You should notice that the flap of metal has been "rolled" upwards on the front fenders. This is exactly what fender rolling is all about. The rear fenders will need to be rolled to make room for wide wheels or low offsets, or else your tires may rub on the metal when you take hard corners or hit bumps. Any experienced wheel/tire shop should know how to roll a fender. If you go to a shop that does not seem like they know what they're doing, get out of there.