Off-pavement wheels for a Gen II Cayenne Turbo
The number of people who'll care about this is probably vanishingly small, but had this information been available to me from the internet, it'd would have saved a lot of time and anxiety. So as a public service . . .
I have a second generation Porsche Cayenne Turbo which I'll use off-pavement. The Turbo's big brake rotors will not allow 18" wheels or smaller. After reading something on the web that implied that the even huger ceramic rotors on the Turbo S wouldn't take 18s but plain Turbo steel brakes might, we borrowed two different 18s to make sure. Offset/backspacing doesn't enter into it; 18's inner diameter won't fit over the rotors.
So if you hope to gain some sidewall height on a newer Turbo, 19" wheels are the best you can do. Sadly, 19s are not a common size, being found mostly on a limited number of German vehicles. Notably, Q7s and Touaregs have optional 19" wheels which will fit (as you'd expect given they're much the same vehicle) so buying off eBay or Craigslist could have worked, though there were relatively few bargains to be had.
The aftermarket didn't offer much, either, particularly anything that looked even a little "off-roady," as opposed to "show-lots-of-caliper" upgrades targeted at youthful Bimmer drivers. But good ol' Tire Rack had the decent-looking and reasonably-priced Sport Edition WX5
in a 19" designed to fit the CTT. So we got a set in "Titanium Gunmetal," which is one seriously fancy name for grey, and they look good and fit just fine.
So on to tires, which wasn't any easier. Most all of the very-limited number of 19s--18s or 20s afford waaay better selections--are SUV street sport tires. Most of the usual all-terrain suspects aren't made in 19s--no Coopers, BFGs, DynaPros, Grabbers, Firestone Destinations, not even my beloved LTX-MS2s (which come in 36 sizes, but not a single 19), etc.--and of the few that were available, most were obviously going to be too big. The final choice came down to the aggressive AT Goodyear Duratracs and unaggressive AT Goodyear Wrangler Adventure. I could have a Duratrac in 255/55R19 or an Adventure in 255/60R19.
What constituted "too big" was a crap shoot. The factory Turbo diameters are in the 29.5" range depending on wheel, but it looked like one could clearly get some more, though no one was clear about how much more. The advice-givers on the internet liked to talk about the 265/65R18 tires on the Transyberrian (which were 31.6 inches in diameter), but that didn't tell me anything since my chassis/body weren't the same. To make a long story short, we gambled on the Adventures, principally to get the extra inch of height (31.1 vs. 30.0) and the higher profile. I have Duratracs on my EarthRoamer Jeep and like them very well on that truck. But besides being smaller, they look very aggressive (though they pavement-drive way better than you'd think from their appearance) and my Turbo was already going to look plenty goofy to the cognoscenti, so that entered into the decision, too.
They fit, but they just barely fit. Make that just BARELY fit. We're talking silvers of daylight visible between the turned front wheels and the fender liner. They look OK with the wheels straight--the tightest point is a bit less than an inch at the front edge of the fronts. But the front clearance gets less as the wheel is turned. I believe that if you get any bigger tire, you'll need to modify the front fender liner. And, no, I am not feeling good about how I'll do in a lot of mud with so little space for it in the fender wells. But they fit and look pretty credible, IMHO.
Now we faced the problem of the big increase in the diameter to deal with, being a whopping 6% over the 295/35R21 rubber bands that came on the truck. The rule of thumb is to avoid anything more than 3% increases to keep the computers to from going weird, but we hoped that if all the wheels were the same diameter, maybe we'd get a pass. And I have the adjustable suspension, which means the tires also have to not foul the bodywork when the suspension is lowered. All praise to the Porsche gods, for we didn't have a problem with either computers or suspension.
So a very happy ending. The tires are quiet and secure on pavement, and if you drive the Cayenne at anything close to socially-acceptable speeds, the handling is still extremely good. Then, too, I took it out to Northwest MogFest yesterday to run around the bumpy fields to see how it worked. It was a tremendous success, and a lot of fun. I've taken a lot of vehicles over lots of washboard surfaces, and this is the most adept. impressively high speeds were possible without drama, as the air suspension and clever chassis electronics seemed to be fully up to the task. Color me delighted . . . along with somewhat surprised that it all turned out so well.
And as a final note: I've used the "we" pronoun several times above. That's because I had the help of Jeremy Williams (mv216v here on ExPo) and his crew at Portland Eurocar shop Matrix Integrated. Jeremy and his staff fully "got" the reasons for the tire change, didn't give me grief for "ruining the Turbo," borrowed the 18" wheels to use as a test, ordered up my tires and wheels and accepted the deliveries, mounted and (perfectly) balanced them, and then drove the Cayenne around enough to make sure the computers weren't going to give me problems and that there was no rubbing. And then, to ice the cake, they cleaned up my original 21" wheel/tire sets and put them in tire bags for me to store. I don't if I'd have gone forward with the new shoes without Matrix's help, but I do know it wouldn't have been nearly so easy. Thanks, Jeremy.
As for length, traveling solo I just move the passenger seat forward to allow for 6'-5". I do this in my bmw 535 wagon though which actually has a touch more usable length than the previous generation Cayenne as seen in those photos. Obviously not on a cot though! It's still more comfortable than an ice cave 
