What are the sidewall ratios on those tires?

 

Duane, I agree with you. Here's an interesting thread from renntech.org from member "tagger" with a 2002 996TT going to 235-40's up front but leaving the 295-30's out back. http://www.renntech.org/forums/lofiv...hp/t18131.html
"Loren" another member replies the following:

"As long as the tires have same N rating are paired on each axle - you should not have a problem. As a rule of thumb you need to be within 4% front to rear. Normally if the system is out of percentage and it is not happy it will throw fault codes. I doubt you far enough off to throw a code."

I have (3) questions/comments:
(1) Here "Loren" says 4% diameter difference is ok, does anyone know exactly what Porsche states in their manual?
(2) Has anyone ever experienced a fault code due to tire diameters being off?
(3) Has anyone ever had their viscous coupling fail with or without the fault code and at what driving condition?

This is such an interesting topic!!

 

Since Porsche only recomends 18 inch wheels and N rated tires on the car they don't give you any information. Bumping up to 19" wheels and keeping the same aspect ratio of 30 will make the over all diameter of the wheel/tire combo to tall. I'd suggest 315x25x19 on the rear.

 

I have had PSM kick in at odd irritating times when i was running tires
that were off by a few percent.

 

The only spec I know of in the workshop manual is for axial and radial runout.

 

I have had similar experiences as Joe mentioned. I just came back from the track where I ran 245/40/18 (Toyo R888) up front and 295/30/18 in the rear and I had intermittent PSM issues. I believe you wouldn't see it on the street under normal/hard driving, only apparent on the track.