Here's some info on fitting non-stockwheel sizes on your Z:
.....Also, 225x50-16 tires on 16x7" 0-offset wheels will fit an early (lowered) model Z with only slight trimming of the rear fender lip. 225X50 15 tires on 15x7 0-offset wheels also fit with no trimming. (been there, raced on both) 205x60-15 tires are a good size for the 15" turbo rims and won't change your speedo 215x60- 15 also fit ok and have only a minor affect on the speedo. 225x50-15 tires WILL fit the 15" turbo rims with just a little bit of pooch.
First a little background. The TRA (Tire and Rim Association) set a 'design' or 'measuring' rim for each tire size, to ensure that all manufactures measure their tire on the same wheel. For 50 series tires and higher, the measuring rim width is 70% of the tire's section width, rounded to the nearest 1/2". For tires lower than a 50 series, the measuring rim is 85% of the tires section width. The actual dimensions of a tire are dependent upon the rim on which it is mounted.
Important note: A tire's width changes about .2" for every 1/2" change in rim width (over or under the 'design' rim). So you can 'stretch' a tire's width with an increase in rim width, within the approved range. Now for popular tire sizes from the fitment guide:
|Tire Size||Rim Width Range||Overall Diameter||Tread Width|
|185/70/14 BFG Radial T/A||5.0"- 6.5"||24.2"||5.5"|
|195/70/14 BFG Radial T/A||5.0 - 7.0||24.7||5.8|
|205/70/14 BFG Radial T/A||5.5-7.5||25.2||6.1|
|185/60/14 BFG Comp T/A HR4||5.0-6.5||22.8||5.8|
|195/60/14 BFG Comp T/A HR4||5.0- 7.0||23.2||6.2|
|215/60/14 BFG Comp T/A HR4||6.0 - 7.5||24.2||6.8|
|225/60/14 BFG Comp T/A HR4||6.0 - 8.0||24.7||7.0|
|205/60/15 BFG Comp T/A HR4||6.0- 7.5||24.7||6.1|
|215/60/15 BFG Comp T/A HR4||6.0- 7.5||25.2||6.8|
|205/50/15 BFG Comp T/A ZR||5.5-7.5||23.1||6.2|
|225/50/15 BFG Comp T/A ZR||6.0- 8.0||23 9||6.8|
|225/50/16 BFG Comp T/A ZR||6.0- 8.0||24.8||6.8|
|245/50/16 BFG Comp T/A ZR||7.0 - 9.5||25.7||7.5|
This fitment info is obviously taken from my BFG manual (I had to become a TEAM T/A member to buy my Comp T/A R1 Road Racing Tires). Other brands of tires may have a slightly different range (so consult your supplier), but these will be pretty close for most. Oh, "tread width" is the actual rubber on the road -- section width (the first number in a tire size, in millimeters) is the widest part of the tire, which is most often the bulge in the middle of the tire. Section width is as stated, but tread width can change significantly from manufacturer to manufacturer.
In my 'archives' I found one other warning; "Never mount a tire on a rim width that is not approved." ". . .wheels narrower or wider than approved> will stress the tire sufficiently to cause poor service, and may even cause the tire to fail."