Rolf - I'm sorry but this just made me laugh out loud. Stowing 5lb of canvas and a bit of magnesium frame a few inches further back causing the inner edges of tyres to wear out :thumbsdown: Overinflating the tyres by 10% :thumbsdown: All that will do is wear the centre strip out.
My post was talking about the retractable roof, not a rag top. The information was given to me by people far more knowledgeable about tires and wheels then you and I will ever be (the wear issue was pointed out to me by a tire shop manager and the extra 10 % pressure by my BMW maintenance manager that is in the business for over 30 years and was trained by BMW in Germany).
So you see it is no laughing matter at all, I hope people with 2009, 2010 and 2011 Z's take the matter seriously.
2.5 degree, chamber, wear on the inside are all facts on our cars.
Once the tire has worn on the inside your main tread is still good, no need to change the tire, just add 10% more air to take advantage of the good tread.
one more thing, do check your air pressure often, I find that I have to add 3 to 5 psi every Monday morning.