Hi,
Hubcentric Rings or Spacers are designed to fill in the gap between the Hub of the Car and the Center Bore of the Wheel. Most Aftermarket Wheel Manufacturers design their Wheels with a Center Bore large enough to fit on many different Cars instead of for a specific application.
Therefore, since Wheel Manufactures make their Center Bore large enough to fit most Cars, most Wheel fitments have a gap between the Hub and the Center Bore. This gap usually doesn't allow for the Wheel to fit Hubcentric but rather Lugcentric which can cause vibration. To fill the gap and ensure the fitment is Hubcentric, Hub Rings or Spacers are used.
One thing you might want to consider though, although 19" Wheels may offer the best aesthetics, they are not usually the best performing size rim. For the Boxster, the 18" rims seem to offer the best compromise between Aesthetics and Performance. It all depends on what you're after. Hope this helps...
Happy Motoring!... Jim'99
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