Originally Posted by Alstann

ktmmike,

You are correct. The base 18" wheel comes from the factory with a taller (higher profile) tire. If we take a look at the rear wheels only, the base model comes with a 245/45-18 tire in the rear, where the sport 19" wheel comes with a 275/35-19 tire in the rear. You will find that these tires are only .1 inch different in overall diameter, or in other words, they are the same height overall.

So, if you were to take a set of base wheels and tires, and slap them on the Sport package equipped Z, the speedometer would be almost exactly identical.