For an 8" rim, you could fit from a 225 to a 245, maybe even a 255.
However, given the Cayennes wieght (5000+ pounds) and potential useage, you would likely want a tire built to withstand the abuse. Be sure to check someone familiar with Cayennes and/or Toureggs about what works best.
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1976 914 2.0
2000 Boxster 2.7 (sold)
1978 911 SC (sold)
1970 914 w/2056 (sold)
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