Torags is correct. All the tire/wheel combinations sold by Porsche for the Cayenne are aproximately 29" in diameter, including the collapsible spare. There will be no need for the air suspension to compensate. Go to tirerack.com and look up the various combinations. Overall diameter is quoted as a spec. The 19" and 20" are actually slightly smaller in diameter than the 18".

I don't know that the redesigned subwoofer is smaller. Remember that the old subwoofer had a battery mounted under it so it was more like a donut where the new one is like a large donut hole. They probably have the same volume.

 

GM is correct. The outside diameter of the collapsible spare (once filled) is in an acceptable range of tire diameters. The subwoofer is also the same, it's the housing that's different (2nd battery vs. no 2nd battery).
J

 

Hmm, maybe I should consider a donut in one of those reasonable-looking vinyl covers, then throw it in the "trunk".

And I've seen this question asked but not answered: could the 2nd battery be removed then?

Anyone know if the new C's don't need a 2nd one because: a) It was never needed or b) They improved either the circuitry or the primary battery?

 

Is it "possible"... well, anything's possible. Just don't know if it's a reasonable job or not. Would need to remove the battery, cables, wiring, and ensure that the electrical system no longer "recognizes" or looks for the 2nd battery. Of course, you could take it out & duct tape the leads and wait for something "bad" to happen.... (kidding)....
No - the 2nd battery was never needed. It's existence was due to two factors: there was an inventory of the units from the supplier (so they "burned through" a bunch, realizing that no one would ever order it as an option), and second, there was a "concern" that the Cayenne would suffer the same fate as other P cars - to be driven infrequently, thus the possibility of run down batteries... and the procedure to "reboot" a Cayenne is a bit of a pain. Trouble is, no one bothered to think this through... if the primary battery dies, where does the power come from to switch to the second? And if the second battery is powering that, then by definition the circuit is closed, and the second battery would also be drained (could be at a lower rate, though...).... In short, it was a triumph of marketing over good engineering.
J

 

Well, I just experienced the unfortunate task of changing my sister-in-laws flat tire. No goop would have helped her even though she drives a regular passenger car. I echo the sentiment that goop will not always do it. Waiting for roadside assistance, would they have to tow you or repair on-site?

I am on the road to Mississippi the end of this month. I have put in a request at my dealer for retrofit. Maybe the Nav system should have a built in tire store locater