bw1999

So, while my car has been in the shop for the past month or so, awaiting the completion of a UMW clutch, flywheel, and some other bits and pieces (thanks Kevin!), I started thinking about winter tires.

There is Rennlister near me (that NEVER happens) who is selling a set of winter tires and wheels. (https://rennlist.com/forums/parts-ma...-set-18-a.html).
My question is his offset is 54/40 (F/R) while I believe the 996 turbo is 50/47 for winter tires.

My question is, is this something that can be fixed with remounting or something, or does this mean the wheels are not usable on this car.

Appreciate your help.

gophaster

Took a quick look at the ad. 996Turbo stock wheels are 8x18 50mm and 11x18 45mm. The wheels you are looking at are 8x18 54mm and 10x18 40mm.

The fronts are almost the same as our stock wheels (they will sit inward inside the fender about 4mm vs stock) this would barely be noticeable..so these should be fine. The rears are 40mm but they are only 10 inches wide so they will sit about 7mm more inward into the fender than our stock wheels. Again, barely noticeable, so should not be a problem.

As for the tires the 235/40 are very slightly larger overall diameter vs stock but it should not be an issue at all. The rears though at 295/35 are about 1.16" larger diameter than stock. I think this is a bit too much of a difference and might not make the awd happy. Not only is this larger than stock but in the stock setup the rear tires are actually supposed to be the same size overall diameter as the front or even just slightly smaller overall diameter than fronts...so this would throw off the ratio. There is some debate as to what (if any) affect this would have on the awd system but I personally wouldn't run that rear size.

Hopefully others will chime in with more info (and to double-check my math)

Macster

Can't speak to the offset question but the wheels/tires are not 996 Turbo compatible based on my info.

The wheels/tires: 235/40x18 and 295/35x18 have a revs per mile number of 794 and 772 respectively. Notice the rear tires make fewer (!) revs per mile than the front.

The stock setup: 225/40x18 and 395/30x18 have a revs per mile number of 804 and 808 respectively. In this case then the rear tires make more (!!) revs per mile than the front.

If you fit the first wheel/tire combo to your Turbo there will be no transfer of torque to the front diff and in fact because the fronts are making more turns per mile the front axle at the viscous coupling will be acting as a brake/drag on the rear diff. You will forgo any benefit of the AWD feature and also increasing drive train HP loss.

Not only is the speed difference a different and opposite sign (negative vs. positive), the speed difference is greater (-2.77% vs. +0.49%) and this can possibly subject the viscous coupling to too much heat and run the coupling and with it the front diff.

My advice is to always consult the factory tire/wheel documentation for your car and find out what the factory sanctions and go with the wheel/tire sizes that agree with the factory.

You don't have to buy the wheels/tires from Porsche but wherever you do buy them the front and rear diameters and revs per mile and percentage differences should be close to the same as would be obtained with factory sanctioned wheels/tires fitted.

bw1999

Thanks so much. I think I understand better now. Will talk to my dealer about winter wheels. I guess I was surprised cause people have said there are so many options on Ebay/craigslist; I didn't realize that most would not fit my car.

Rustler

As Gophaster said the greater offset on the wheels you are looking at will move the mounting point of the wheel farther outboard on the wheel drum, which moves the wheel farther inward in the wheel well, essentially the wheels will look more "sunken". You could run spacers which would effectively push the wheel farther out in the wheel well, making them more flush with the fender.

The factor recommended winter wheel size is 225/40-18 (F) and 265/35-18 (R). That makes the rear .8% LARGER than the front (804 revs front v. 797 revs rear). There is a lot of speculation about whether the fronts should be slightly larger than rears as on the stock summer tire setup (front is .4% larger on stock summer setup), but I have trouble believing that Porsche would have a factory spec winter wheels size that damages the AWD coupling or negates the efficacy of the AWD system.

An alternative to the factory winter tire spec above is 235/40-18 (F) and 275/35-18 (R). That is both a size recommended by tire rack and I ran it by the service dept. manager at a Porsche dealer who gave it a thumbs up.

Even with those two combos, finding a winter set can be a challenge, Good luck!

stefn1

Can anyone confirm this 235/40/18 and 275/35/18 size?

cannot find all seasons or winters to fit on stock 18x8 and 18x11 rims in stock sizes which are 225/40/18 and 295/30/18

I don't want a more narrow rim as it wont be seeing snow, just winter temps.

Thanks

splitime

If it helps, I'm running a 10" rear on my setup (narrowbody wheels/offset) with a 14mm spacer(fairly common spacer for our cars with stock wheels). Fronts are almost stock fitment otherwise.

Tire size that I've successfully run a winter on are:
Fronts - 225/40/18
Rears - 265/35/18

Blizzak LM-60

stefn1

thanks for the info. more interested to see what sizes guys downsize to while still using 18x8 and 18x11 rims.

stefn1

anyone?