Tim,
I guess the answer is that there is no reason in the world why you can't fit 18" or 19" wheels to your 964. Whether such a fitment is approved or advisable is entirely another matter.
The subject has been beaten to death over the past years (particularly on Rennlist) but the issue of rolling radius is not the whole story. The larger diameter wheels in themselves weigh more than the OEM 16" fitment. In addition, the larger diameter requires a lower aspect ratio tyre to maintain the original rolling radius and these lower aspect ratio tyres have lower and stiffer sidewalls. The stiffer sidewalls impart additional forces to the wheels, and hence to the steering and suspension components. Invariably, the larger wheels are a wider cross section than the original OEM parts so serve to increase the forces yet further.
Apparently, from what I can gather, through Rennlist particularly, it was Porsche North America which issued the Technical Bulletin saying that 18" wheels (specifically) were not approved for fitment to the 964 and the general feeling is that this was done for legal reasons primariliy to preclude legal action again PCNA in the event of some catastrophe befalling a car with 18" wheels fitted.
Apparently neither Porsche AG nor Porsche GB have ever issued such a bulletin.
There was an issue with the Porsche Cup cars in, in believe, 1992, where they were fitted with 18" wheels and slick tyres and they suffered a few failures of stub axles attributed to the additional forces of the larger wheels. Porsche reverted to 17" wheels to overcome the problem. However, that being said, the forces imparted to the suspension and steering through slick tyres on 18" wheels in a racing environment is considerably more than you would ever expect to be experienced by a road car on regular road tyres.
Adrian Streather has always been an advocate of NOT fitting 18" wheels to the 964 but I noticed that even HE has succumbed in having 19" wheels fitted to his car recently. I have no idea what other modicfications have been made to his car to accommodate 19" wheels (if any).
Apart from the usual concerns of tyre rub on the chassis and/or wings there seems to be a general acceptance that fitting 18" wheels improves the appearance of the 964. I have no personal experience of 19" wheels although one can only assume that if 18" wheels are bad, 19" wheels must be worse.
Personally, I'm very happy with the 18" wheels fitted to my 964 - and the wheels haven't fallen off yet!!
Best regards
Dave
