Joe Cool

I purchased a set of used slicks for an upcoming track day. The fronts are 235/645/19 and the rear are 305/690/19. The tires do not rub at all but have a different size ratio compared to stock. The rear stock tires are about 3.5% larger in diameter than the front. These tires are closer to 7% larger. When I gently drove the car around the block to make sure there was no rubbing, I got numerous alarms on my dash indicating faults with the ABS, PSM and PASM. When I got home I reset the codes and went around the block again, this time without throwing any codes. Despite that, the red exclamation point on the tach and a message that read system fault visit service. While I am not concerned specifically about the different ratios of these tires compared to stock, I am very concerned about the possibility of damaging the viscous coupler for the all-wheel-drive system. My question is, should I be concerned or am I worrying too much? Also, is there a way to recalibrate the wheel speed sensors so the ecu will tolerate the new tires?

Dennis C

Not sure if this helps, but the acceptable variance that is generally used for the 996 viscous coupling is 3%. Anything greater than that would concern me.

Joe Cool

Thanks for the reply, Dennis.

Do you have any Porsche technical data that defines this limitation? I don't doubt it's correct, I just hope to find something specific from Porsche.

the_buch

It's my understanding that this much difference in wheel rpms registered from front to back will be considered by the various systems to be indicative of slippage in traction causing significant redistribution of power to the coupler. I don't think you can have that big a spread with this simple 4wd tech?

Joe Cool

That's my understanding as well. Thanks

Dennis C

Unfortunately, I don't have any official documentation. I'm just passing on what I've read on these forums.

Joe Cool

No worries, thanks anyway