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I had a profile photo and decided to change it. The upload photo option did not work, exactly as described in original post above. I tried Chrome, IE-32, IE-64 all same. Restarted PC, browsers, no success. Finally I placed the new photo at a temporary URL and was able to load from the link. Tried to upload directly again to see if something had reset, same result it only uploads from URL. FYI if someone else has similar problems.

Hi dphatch and qikqbn , sorry I didn't see your post sooner, here’s what info I have anyway, might be useful for someone else. I had the same question when shopping for winter tires for my 2007 C4S. Basically, there is a given ratio of front/rear rotations (or circumferences or diameters or revolutions per mile.) If that ratio is changed it is like your front or back wheels were slipping on ice. So, to find the difference or ratio you can use an online tire size calculator such as http://www.tyresizecalculator.com/ (or you can just use the diameter difference that you already calculated. 26.4-25.5=0.9”, 0.9/25.5=0.0353 =3.53%) I’m pasting below the results from the calculator which are similar with a little more precision. In either case it exceeds the 3% “slippage” mentioned to me by one Porsche technician and the 1.5% mentioned by the calculator. In the case of 245-35-19 the difference is 1.2% so acceptable.

Hey Seahawkeye, How do you like your OZs?? I really think they are the absolute best value in high performance wheels. That finish is as tough as nails. Racerken is absolutely right. These fat wheels collect all kinds of crap in the winter. Way more than enough to throw the wheels out of balance. Your teeth will rattle at 80 MPH. After a messy trip I power wash the wheels right in my garage just to blow all the crap away. With a power washer it does not take much water to do this. I got BBS SR Anthracite wheels. The fat wheels immediately accumulated snow and ice on the down sides when parked outside. And sure enough the unbalanced shaking started at about 40 mph. My low tech solution was to scoop out some of it with my hand - good to keep old gloves around. No unbalanced wheel feel after that at legal highway speeds anyway. While I have water and power wash hooked in my home garage it's too tight so I can only use it outside when its above freezing which isn't often lately. There's a no touch car wash with undercarriage wash option not far off my route so I go there as soon as I am back from a long salty drive.

Thanks for the input Mijostyn and Racerken. I waited to have the 18" wheels with Michelin PA2 235 front and 265 rear installed and a chance to drive a bit before posting my impressions. (I was leaning Michelin to begin with and Sottozero 295 were nowhere available anyway.) We've had some good storms lately in which I had to drive so I think they got a fair shake. To recap: The main question was for a 2007 C4S, whether rear 265 is OK even though the owner's manual only allows 295 in rear (see page 332). The short answer was yes because I was able to obtain a document from Porsche NA stating so. I'll be happy to email the document to anyone else who faces the conundrum. (The owner's manual already lists 265 for rear on C2S (see page 330), so that wasn't the question. The question had -I think- further importance for a C4S as the front/back turning ratios might affect the AWD, which was also laid to rest -I think. I've subsequently wondered what the new tires option in the computer does and whether it does some related measuring.) My impressions after about 400 miles on wet/slush/ice highway and 100 miles state roads on snow/slush in heavy storm: No surprises. Handling: 18" definitely feels softer in cornering and when doing a quick jig around a pothole. More rally less track feel. But that was the tradeoff I expected and wanted to make for a softer ride on wintery uneven surfaces with minor potholes. Happy. Grip: Grip improved visibly as the tires wore in. Impressive grip more than I at first expected upon limit braking and cornering, with very decent initial grip for hill starting, and predictable gradual letting go at the threshold. I didn't push it anywhere near the point of planing on slush but it felt solidly planted during the occasional overtake requiring crossing the middle slush barrier that forms between two lanes on the highway. Happy. Balance: On dry cold pavement the car is obviously more tail happy with the 265, though at colder temps I fear the summer 305 might have hardened and let go very suddenly and I didn't push to find out. On wet/snow/ice the 265 is tail happy but progressive as the tail breaks away then less so as the power gets transferred to the front. Again as expected, and again as I wanted it. (Having spent formative years and winters in an old Beetle I am happy with plenty oversteer, but prefer it not be the jack knife variety while I'm tired after a long day of skiing. Also a function of age I sadly admit.) PS. I agree undercarriage washing can only be good to reduce corrosion. I wonder if most of the damage is already done during driving when the parts are hot and might react and corrode more quickly? Google search for "steel salt corrosion temperature" brings up a bunch of data showing temperature does matter though not so obvious how much it matters. I guess the corrosion question is worthy of its own forum thread. PPS. I'd also like to give a thumbs up to both Tire Rack as well as My Tire Guys, who made the delivery and installation work among considerable uncertainty and inclement weather. Happy safe and sensual winter driving to all.

OK if anyone is reading this thread at all, I'll share what I did get after speaking directly with every party involved. (This is all only for 2007 C4S rear winter tires on 18" wheel, and I am dispensing with profile options.) Porsche Owner's Manual lists only 295 for rear winter tire, and Tire Rack lists 265, per my previous post. Michelin said only 295 will do. Porsche Dealer#1 said only 295 will do. Porsche Dealer#2 said 295 as well as 265 will do. (Also said the AWD/PSM/ABS tolerances in "tire slip" is 3%. My calculation gives a difference <1% between 265/295 per posting above. Whether 265 in rear results in slightly more power being sent continually to front is an interesting question.) Porsche North America said 265 is correct, and emailed me document to back it up so I am assured. Conclusion: Whatever your owner's manual says, check for more up to date info if it doesn't immediately click.

I also bought a CPO 2007 997 C4S coupe recently and am in the market for winter tires now. My choice tires are Michelin Pilot Alpin PA2 N2 all around on 18" wheels. I drive regularly in MA,NH,VT and often enough in fresh snow, so I understand that a smaller footprint will cut deeper in snow, but will sacrifice on pavement and we do get black ice on highways often enough. The Owner's Manual only lists 235/40 front and 295/35 rear snow tires for 18" wheels on a C4S. (for a CS it lists 235/40 front and 265/40 rear) Tire Rack online as well as in conversation, recommends 265/40 rear for C4S and also for CS. (In fact the 295 aren't even available in matching N numbers but that's an inventory and money headache.) I went through a circumference calculator with the following numbers for tires on wheels: 235/35 on 19" circumference is 80.037" 305/30 on 19" circumference is 82.325" (I did this to understand what difference there is front/back on my factory installed summer tires, F/B difference in summer is 2.288") 235/35 on 18" circumference is 79.802" 295/35 on 18" circumference is 82.090" <- Porsche recommended. F/B difference 2.288" same for summer and winter 265/40 on 18" circumference is 82.770" <- Tire Rack recommended. F/B difference 2.968" So, the difference between Porsche recommended and Tire Rack recommended on a C4S for winter is 0.680" or about 1.74cm per turn of the rear wheel. This leaves me with the following questions (and maybe some others I don't know to ask): Mechanical: Is the 265 rear tire going to cause trouble for the AWD? The ABS? Could this cause wear in AWD as the F/B ratios will be operating differently from their default most of the time? Handling: What will it mean for handling on ice? Given that the CS uses the 265 rear winter tire and the weight differences are not that huge (more front weight for the front diff) I am somewhat accepting that the ice handling won't be very different from a C2S, am I missing something? I hope someone can shed more light, especially anyone who might be using 265 winter tires in rear of a C4S, thanks.