I posted this on the PCA tech board because I wasn't sure how important changes to the wheel offset/tire OD were to the handling of the car .
Here is Scott's response:Your question posted on PCA.org has been answered:
Classification: Wheels and Tires
Subject: 987 wheel/tire dilema
Model:
Year: 2006
Total Mileage: 300
Car Use: Autocross/track
Question:
I've just recently taken delivery of an '06' Boxster with options to enhance track and auto-x driving--PASM, Sport Seats, Sport Chrono.
I don't want to abuse the stock wheel/tires to auto-x or DE's since it dramatically shortens the tire life. I'm looking for another set or two of tires--one for auto-x the other for DE.
The problem that I've run into is that the 987 has different OD sizes relative to our old '99' and there aren't a lot of tires in R compound sizes to fit the 987.
How im portant is it to maintain the standard tire sizes, wheel offsets and OD to the balance of the car? There are after market tires and wheels that are close, but how close do I need to be?
Thanks for any help
Mike
Answer:
The tire sizes must stay within 5% of the stock sizes OD and front to rear size relationship. This is do to the allowable tolerances for the ABS/PSM and or the PASM system. With the PASM system this is more critical and doesn't allow much room for error. Any other deviations will result in the improper functioning of these systems and impede the handling of the car.
I thought it might affect the ABS/PSM but handn't thought of PASM; YMMV
