I have 17" on my car, and yes, axel's right, if you are concerned with tire price, stay stock size. I just replaced my tires at over $640 and they were the cheapest they had.
There is a formula for tire sizing that will not offset your speedometer, but it's easier to go to some place like discounttiredirect.com and "shop" for a set of their wheel and tire combos. The site will give you the recommended tire size with any wheel size you choose.
As far as lowering is concerned. You shouldn't need to worry about the tires rubbing if you stay with the recommended sizes. We all know the 240sx came with the most awful fender gap at its stock configuration. I am lowered with the Eibach Pro-kit and most people think my ride height is stock because of the stock off-roading set-up I started with.
