Need advice on non-standard rear winter tire sizing for '02 996 C4 (NB)

Hello all! I just took delivery of my first P-car, a 2002 996 C4 Cab! I was originally planning on getting a second set of wheels and winter tires for it next fall, but the seller I ended up buying from already has winter tires on car! That's a bonus for me since I'll be able to start using it as my DD sooner, but there's a big catch: the rears need to be replaced immediately even though at first glance they seem to have TONS of tread left (the car hadn't been aligned for a long time and the inner sidewalls are so worn you can't even read the text on them anymore). So, that leaves me with two good fronts with plenty of tread (Blizzak WS-70, 225/40/R18) and two bad rears (Blizzak LM-60, 265/35/R18). Here's my problem: Bridgestone no longer makes winter tires in 265/35/R18 (the Porsche recommended winter size), not in LM-32, not in LM-60, and not in any of the WS versions. Now my question: would LM-32 rears in 275/35/R18 work without messing up the AWD? My thinking is that because Porsche officially recommends the same front size for both summer and winter (225/40/R18) and... Rear Winter: 265/35/R18 Rear Summer: 285/30/R18 ...wouldn't 275/35/R18 work because it's so close to the summer and winter sizes? Here's the output from https://www.myturbodiesel.com/1000q/...calculator.htm : I'm pretty new at reading this kind of stuff, but it looks like the result on 18x10 wheels is VERY similar, just slightly more sidewall height and circumference. Any thoughts? Is there someone I could contact who would be able to plug all the numbers into a calculator of some sort to make sure it wouldn't mess up the AWD? Thanks!

As an update, I spent almost the entirety of this afternoon researching this topic. What I learned was extremely informative: One of the biggest things I learned was that the tire I originally mentioned above wouldn't have worked after all. The "275" number sounds like it is an in-between of the 265 winter and 285 summer Porsche recommendations, but once all of the details are plugged into a calculator turns out the 275 has a larger circumference than either the 265 or 285! The other thing I learned was thanks to a fascinating and highly-technical discussion I found in another thread: the AWD system in our cars only works properly when the rear tire is just slightly smaller than the front! If it's slightly smaller, it spins a few revs/mile faster and thus "pre-loads" the viscous coupler so that it's ready to send power forward when needed (the summer OEM size is about 1.4% revs/mile faster in the rear than the front). The head-scratching thing about that whole concept is that while winter OEM size is better in the sense of offering a smaller contact patch, it is a slightly larger/slower tire and thus hampers the effectiveness of the AWD! Near as anyone in that thread can figure the reason for this backwards OEM recommendation is either A) there wasn't a better winter tire size on the market to recommend or B) Porsche favored the smaller contact patch as a priority over the AWD system, or both. So, all that to say I think I've finally found a workable tire option! The Michelin Alpin PA4 comes in a size that's actually between the summer and winter OEM circumferences and ever-so-slightly smaller than the front circumference: 255/35/R18 What do you all think? Does this seem like a workable option?

225/40/18 265/35/18 or 235/40/18 275/35/18. Both combos work but 265/35/18 have been impossible to get recently. Been using both combos...now 235/275 only as 265 hard to get....past 7 winters philly to reading. These 2 combos are the perfect size for 18's

Quotes source post: 15504260

Thanks geetee! Glad to hear those combos have been working so well! Have you had any situations where you've been able to discern power being sent to the front wheels? Has it done anything odd like lagged or sent the power forward somewhat suddenly? (those are some of the results that were predicted with larger rears in the other thread I linked to) If I wasn't trying to keep the existing fronts I'd definitely be considering your 235/275 combo despite the size ratio for the AWD, but I'm currently leaning towards those Michelins in 255/35 as long as I can use these charts to convince my local mechanic that I haven't "downsized" my tires and thus disqualified myself from passing that pesky PA inspection.

you're reading too much into the 996 gen awd system...you will not feel the transition of power to the fronts. the % is way too minimal anyways. Yes ideally you want a slightly smaller diameter rear in theory but does not really matter as long as you're within I believe 3% diameter difference too not burn up your front diff. 996 awd is nothing like an audi or Subaru....way way less % sent to the fronts on a 10 wide wheel 255 35 18 is fine since when did PA inspection check tire size? Never had an issue with that as even my summer tires are nonstock size

@geetee haha, yeah, maybe I'm overthinking it, but I'd like to avoid replacing all 4 right now, I don't want to damage anything, and I'd still like to get something out of my AWD even if it's nowhere close to a proper system like those others. Thanks for confirming my thoughts on the 255/35R18. Maybe my mechanic is just more thorough than most, but when I called to pick his brain about tire sizes he mentioned that downsized ones were a problem. I dug into the regs and found this one that would seem to back him up: 067 Pa. Code 175.80 (e) Beneath the vehicle inspection. A beneath the vehicle inspection shall be performed as follows: (1) Inspect the tires and wheels and reject if one or more of the following apply: (xxii) A tire is smaller than the manufacturer's recommended minimum size or below the manufacturers recommended load rating.

Let me blow your mind a little bit: Notice the 17 inch sizes? https://files1.porsche.com/filestore...t-classics.pdf

That's interesting about the pa code. I get the load rating for safety and we are very **** heavy....you can't skip on the load rating but "tire is smaller"? By smaller do they mean overall diameter? width? 996 is 17 or 18 size factory. I've had 19's at one point that passed. I currently have 1 car with 1/2 inch taller tires and 20mm wider tires that pass...1 car that has 20mm wider but 1/2 inch diameter smaller tire that also pass. The term "tire is smaller" is a little vague...but I guess it depends on who you go to for inspection....

Quotes source post: 15504761

Ever stare straight at something and never see it?!?! I've spent hours pouring over those charts, plugging numbers into the calculators to see if I'd be better off buying 17" wheels and tires for the winter, pricing out all those various options... ...and never once did it occur to me that "you know, the 18 wheels on it are a factory upgrade over the normal 17s" would be my ace-in-the-hole for any "downsizing" objections to getting past the PA inspection!! Thanks!!!

Quotes source post: 15504768

Yeah, I thought it was strangely vague as well, but between the actual circumferences of the 255/35 vs the 265 or 285 and @strathconaman 's stunningly-obvious point about the 17" standard wheels, I think I should be pretty safe for the inspection.

Hey what are people running for winter tires these days? Looking for suggestions for a 2002 c4 convertible. In Toronto.

Sorry total newb here. Would these fit a 2002 Carrera 4 convertible?

Quotes source post: 18508232

Yes they will

Thanks! Is this an ideal combo? Or would 265 preferably be paired with the 235s?

Quotes source post: 18508347

It is the porsche recommended spec. I don't think wider necessarily works better on a winter tire.

Thanks, What size rims do I need for these?

Would they fit on these? 265s on 10s vs 9.5s?

Yes 265 fit on 10s

Quotes source post: 18508784

Thanks! And the et47 offset is what I want?

Quotes source post: 18508800

Found a set on Kijiji that looks interesting: �Selling Porsche Carrera 911 (996) �Turbo Look II� (Turbo Twist) 18 Inch Wheels and Tires These wheels were originally part of an official winter setup offered by Porsche for wide-body 996 911 vehicles under the option code M415. These wheels were never offered on any Porsche vehicle - they could only be ordered via a Porsche dealer as part of a 996 WB winter setup. Very few sets were produced and are made by BBS! Front: Size: 18x8 Offset: ET50 Tires: 225/40/18 Part Number: 996.362.136.01 Rear: Size: 18x10 Offset: ET47 Tires: 265/35/18 Part Number: 996.362.140.01� Would these also be a workable option? Sorry for all the questions. Totally new to this. Mostly confused about the offset aspect. Thanks for all the help.

Quotes source post: 18509282

Those rears will stick out 18mm farther than your current wheels. Sorry I didn�t read that you were looking for wheels too. I figured you are mounting winter tires to your stock rims. In the table I posted it lists the rim size you need. Not the 4s sizes. 18x10 65 for the rear

Quotes source post: 18509301

Thanks! I am fine with them sticking out 18mm from an appearance standpoint. Is 18mm ok from a technical/mechanical/awd perspective?

Quotes source post: 18508204

I did extensive research and calculations to make sure the ratios would work (or at least not cripple/damage) the vicious coupler on my 2002 C4 and eventually settled on: Front: Blizzak WS70 225/40R18 Rear: Pilot Aplin PA4 255/35R18 (the model mismatch is because of a badly-botched alignment under the previous owner so that the rears were in desperate need of replacement when I acquired the car while the fronts still had plenty of life left) I haven't done any direct testing, so I can't guarantee the combination, but it's been my impression that it seems to work well on my car.

I am running the recommended Winter setup, stock 17" with 205/50 and 255/40 Pirelli SottoZero's, with zero issues. Gives me and my C4 Cab nice footing in the snow and slush.