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08 C4S tire and wheel fitment

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Factory spec is 19x11 and 19x8 with 305/30 rears and 235/35/19 front and snow option for 18x8 and 18x11 is 295/35 and 235/40/18. I really want and all season setup, no dedicated snow tires. Mostly winter driving in dry and wet, little snow or ice as I use an SUV the worst several days. 295/35/18 rears would be ideal, but tire rack has the Conti DWS06 I want on back-order, so looking for another size option, keeping the front at 235/40/18.


The stock rear to front diameter difference is rear +0.7 inch, which is same for 18 inch wheel factory recommendations.


Can I choose any rear tire that fits the rear rim size correctly as long as the rear diameter stays between 0 inch and 0.7 inch more than the front and still keep the AWD and electronic aids all happy? Do I have to stay at stock 0.7 inch height difference? If not, how far can I go up or down in rear diameter, 1% or 1.5%? 


For example, can I use rear 265/40/18 or 285/35/18 or 275/35/18 with front 235/40/18? There are better high performance options in these sizes, but my dealer won't commit to anything other than factory(I understand). The guy at the indy shop I will use seems to thinks as long as the rear is the same height as front or up to the factory .7 inch taller than front, and the car should be ok in terms of systems not going crazy. I understand I will lose some ideal handling characteristics, but this is winter daily driving, not aggressive at all, and I have the 19 MPSS's for the other 3 seasons of more fun. Any insights would help.


For what it's worth, I would be getting 18x10 and 18x8 winter rims from tire rack. Also the stock carrera and S snow sizes of 235/40/18 and 265/40/18 I believe. Since I am only using wheels that fit with correct offsets, smaller tires should be ok too. 


Last option is I might can source 295/30/19 which is only about 0.2 inch shorter than stock 305/30, or 0.5 inch taller than front 235/35 stock size. But sourcing that 295/30 would require getting tires and wheels from 2 locations instead of the nice tidy all in one option I would like, plus narrower is just better when roads get a little slick.   Thanks in advance.

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Hi John,


Your indy shop's advice is correct.  We work with aftermarket fitments on these cars up to 20",  and they have proven to be quite tolearant of slight variations on OD.  Typically, we try to respect the factory stagger (typical cars have none, but hen again Porsche are not typical cars!).  In the case of your C4S the factory 235/35-18 (25.48 OD) and 305/30-19 (26.20 OD) has a factory stagger of 2.8%.  The 235/40-18 (25.4) and 285/35-18 (25.86) yields a 1.8% stagger.  I would suggest you consider a 295/35-18 (26.13) which combined with the 235/40 will give you the exact factory stagger of 2.8%.


That being said so long as you keep the OD difference to less than 1% or so, the cars run just fine.  As a matter of fact we had one by running  245/30-20 (25.79 OD) and 315/25-20 (26.20 OD) a 1.6% difference, and client reported no trouble or lights.


Good luck and enjoy!


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