GRUWEZ

Hello everybody,

Before I start, this is probably a question that has been asked quite a few times before. I searched the archives, but could not really get a conclusive answer to my questions.

I would like to install on my 1989 928 S4 four new wheels. I would either go for 17" Cup II wheels or 17" or 18" Turbo wheels.

After having taken a look on 928gt.com I found some information.

1. Cup II: Wheel size 7.5" x 17" front and 9" x 17" rear. Tires: 225/45ZR17 front and 255/40ZR17 rear.

Question: Which offset (or "ET") do I need in order to make those wheels fit perfectly without having to apply modifications to the fenders or having to install spacers?

2. Turbo: Wheel size 7.5" x 17" front and 9" x 17" rear. Tires: 225/45ZR17 front and 255/40ZR17 rear

OR

Wheel size 8.5" x 18" front and 10" x 18" rear. Tires: 225/40ZR18 front and 265/35ZR18 rear.

Question 1: (same as above) Which offset (or "ET") do I need in order to make those wheels (17" and 18") fit perfectly without having to apply modifications to the fenders or having to install spacers?

Question 2: Do the 18" wheels fit OK? Are there any known clearance issues using 18" wheels of the above mentioned size on a stock 1989 928S4?

Thanks for the feedback,
Rik Gruwez
Brugge, Belgium

Garth S

Aside from offset, fit is largely determined by the tire size, not the wheel, namely overall diameter and section width (NOT tread width).
For the tire sizes mentioned, overall diameter of the 17s to stock 16s is only 0.2" more in the rear, and the 18s are 0.4" larger front and rear (using 24.9" as an average 16" tire diameter). These changes are insignificant at stock ride heights.
Tire section width* measured on a specific rim width is generally of more concern: The 17s and 18s mentioned are marginally narrower than a stock 225/50 R16 at the front (@9.2"), which is good re. fit: the rears are 0.3 and 0.6" wider respectively than a 245/45R16 (@9.9") which should pose no problem - lots of space in the rear wells).
So now that you know the whole black round package will fit underneath like stock - we come to ET, or offset: Here, I will only offer my 'knowns', and the experience of others may differ, or be more extensive.
The front (CS) wheel 8x16, ET 60mm with a 225/50 fits and tracks perfectly. ETs of < 50 may violate the scrub radius /self-centering aspect of the suspension geometry and cause handling issues: the 8.5x18 mentioned I would think should have a minimum ET 55, preferably 60 - but that is only opinion.
In the rear, there is variance car to car - and through the model years: here are two reference points. On my 16v, a max fit is on a 9x16, ET 60 with a 28mm spacer and a 245/45. That is an effective ET of 32mm; were the tire section larger, 1/2 of the section increase would have to be added to the net ET allowable. On the S4, a 245/45 on a 8x16" at ET52.3 works with a 21mm spacer (net ET 31.3mm).
This may give you some reference points , so bring out the calculator .... .

* section width - widest measure of a mounted, inflated tire on a specific rim width: ~mid height on the sidewall.

GlenL

I bet your quest for "perfect" fit has scared off lesser kibitzers, but I'm not so sqeamish:

65mm front, 55mm rear for the 7.5's and 9's

For 8.5's and 10's I suggest searching a bit more.

Of course I'm guessing a bit and extraploting from what I see on my car. But those are the numbers I'd order and I've been looking for 8's and 9's myself. The fron number is important to keep the scrub radius correct for the suspension geometry. That rear offset may be a bit much, but spacers can bring it towards the lip.

A must read on this is from out Australian bretheren:

http://www.landsharkoz.com/tt/ttwheels.htm

Perhaps when the rest of the American continent wakes up you'll get more info!

Looking at your tire numbers, those are all too tall. You've got to reduce the aspect (middle number) to take into account the taller rim. Mr. Chadwick for Australia has handy tables and formulas for doing this.

GlenL

Want to add in something, and Garth has gotten to it, too: Even after picking the wheels, if you're testing the limit of fitment, then the tires you pick will be a huge factor in it's success.

Garth S

Rik, allow me to drop in a specific question - for I have far more concern for the front than the rear.
One of the 993/996 stock front wheels shows as 7.5x18 ET 50 , available in several styles (Cup I,II,IIIII ....), normally with a 225/40R18.
Does anyone here successfully run this on the front?? Does it track well on irregular pavement - or want to 'tramline', and squirt sideways off line??

blau928

Check my sig,, runs fine, and no rubs. needs a 1/4 inch in rear, and car is at stock ride height.

blau928

oh, 18x8et50f, 18x10et65r