The 16" wheels were the standard wheels and the 17" where the optional wheels from day one. That said most were ordered with the 17"s. In 1998 18" wheels also became optional. I'm not sure if or when the 16" wheels were dropped as the standard. I picked up a used set to use with my snow tires.
Last time I checked the base model still comes with 16's as standard equipment, however most are optioned with the 17's. It's rare to see a Boxster rolling on the factory 16 inchers.
It would be intersting to hear from people who have experience regarding the difference in handling between the 16" and 17" wheels with summer tyres. Sometimes, bigger is not always better....
My 1998 came with 16s originally. When I started performance driving I mounted a pair of 17s so I could max my tire widths for stock class. I think the widest recommended tire on a 16x7 wheel is 225. Otherwise the car handled just fine with 16s and tire costs were less.
16 wheels are the trick setup for a base Boxster with snow tires because the larger sidewall gives better pothole protection, 16 snow tires are a lot cheaper and a narrower tire actually offers better traction in the snow.
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2009 Cayman 2.9L PDK (with a few tweaks)
PCA-GPX Chief Driving Instructor-Ret.