Necropost...since this is still such a relevant topic and one of the best overall threads on the issues (I've found).

Can someone PLEASE chime in and tell me if the below tire combo's would be ok/good with my AWD 996 TT on 19" rims?

Any worry of rubbing with these heights? Ratio's of front to rear acceptable? Thank you for your help.

Anyone have any input on the best aggressive offsets for flush-fit 19's? Thank you!!!!

 

I think the prevailing thought is that you want the fronts to actually be larger than the rears if you are still AWD.

 

Thank you.

Hmm, that's what I thought as well...yet I'm being recommended those sizes I posted above. I had my concerns/reservations, which is why I fact checked it here.

Such a PITA. I may consider going RWD someday...

 

What size wheels are you running? SUmaverick is right...you should lean more towards slightly larger diameter in the front in order to keep a more factory ratio.

STOCK
225/40/18 overall diameter: 25.09 inches
295/30/18 overal diameter: 24.97 inches

So stock overall diameter ratio is almost exactly the same overall diameter front and rear. Perhaps ever so slightly smaller diameter rear.
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I actually went with OEM width and profiles front and rear to be 100% that awd is happy

Same tire specs as above but 19" wheels (this is what I am running)
225/40/19 overall diameter: 26.09 inches
295/30/19 overall diameter: 25.97 inches

With above setup you have the exact same ratio front to rear as STOCK. Also, the same ever so slightly smaller diameter rear. This setup matches OEM ratio front to rear so no chance of awd issues at all.
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From the two choices you listed I guess I would go with the 235/295. I think there are lots of guys that are having no issues with either of the setups you are considering though. I just wanted to be really safe and my car is a street car anyway. I hope to do another set of "track" dedicated wheels/tires if I ever do get to do some DE's.

Good luck

 

For reference, I run a 235/35/19 f and a 315/25/19 r on my car

 

I've been running the following setup for the last 2 years with 0 issues.
Front: 245/35/19 on 19x8.5 rim 45 offset
Rears: 305/30/19 on 19x11 rim 52 offset & 15mm spacer.
(except had to roll fenders in the rear)

 

Hi there, thanks for the feedback!

I am currently running 18"rims...8.5 and 11"...with OEM tire sizes. I have an offer for a set of 19's that would be a nice set of street "bling" rims, so I'm simply looking into tire options to see what's viable.

The 19's I'm looking at are 8.5" front with 11" rear.

It would seem your 225/295 suggestion is the most true to MFG recommendations...

Considering this, it's so perplexing why I get so many other suggestions (even from vendors). It's strange to hear so many guys running other combo's and state they have no issues. Does that mean, no issues at the moment...but the diff is definitely being stressed and will eventually wear out due to it?

Anyhow, thanks again...

 

AWD? What tires?

 

Yep, I know that was the previous "perfect" setup for 19's...but too bad there are literally NO good 315/25/19's available in the market.

What tires are you running? Nitto Invo's?

 

Yes, and MPSS's
At least 2 years and 0 issues, this was the combo that was preferred when I was researching for 19's. I didn't want to go with the 315/25 because I live in the NY/NJ metro area and the roads aren't as good as other parts of the country... I wanted the meatiest tire for best ride and handling and not trip any lights on psm.

 

Thanks man.

Ok, so approximately...you are running a .5" taller rear than front.

Can someone please tell me (if he's so far out of factory spec), why guys like this are apparently having no issues after years of driving?

 

Its been said there is a theoretical minimum 3% difference in front to rear diameter or rev/mile ratio that is allowable before you start to constantly trigger and damage the diff (assuming it hasn't already been damaged and is still functional; there is a debate on whether these diffs even work on most of our 10-15 yr old cars). I assume they mean that above this supposed delta difference you're engaging the diff so much that it will eventually heat up and wear or out.
But what causes the psm to trigger. First let me say the "awd" system on the 996 sucks and is not a REAL awd system. Its meant more as a safety system to keep the average joe from wrapping their car around every tree and telephone pole, remember why the old 930's were called the widow makers. My understanding is that it sends 5 to 40% max traction to the front tires and that 40% max is only available at high speeds. The faster you go the more traction can go to the front. But at low speeds the car is basically a RWD vehicle. That being said it means that to get the front wheels to engage and get traction you have to get the rear wheels to spin significantly more than the fronts (i.e. more revs) like when you are peeling out(go give it a shot and watch the lights on ur dash lite up).
So what does that have to do with tire sizes? The stock tire diameters are very close, actually I believe the rears are slightly smaller in diameter, something like 807 revs/mile rear vs. 803 revs/mile front (larger revs/mile mean smaller diameter). Well if you go with too small a diameter rear compared to the fronts chances are you are going to trigger the psm and engage the diff more frequently and if the delta is more than that 3% you will probably eventually burn out the front diff completely. At the same time, if you go with a larger rear tire, chances are you are going to engage the front tires less depending on how much larger the rears are.
Now I think the mistake most people are making when calculating the delta difference percentage is that they are taking the absolute value of front to back difference when they should be keeping the negative vs positive delta difference.
I believe the equation people have been using to help determine if they are within that supposed 3% calculated delta difference is ((rear - front)/front)x100 = delta difference percentage. But if that delta difference is a negative than you're running larger rears and IMO you are probably engaging the front tires even less than on a stock setup.

I could be completely wrong but this is how I justified going with my current tire setup and why I dont think I have ever seen an issue with PSM, but then again it may not even be working . Whenever I do a hard launch I can feel the rear tires breaking loose and the traction ! light comes up on the dash, but under normal driving I have never seen it flash, even with psm off.
I may be over simplifying the whole awd diff system, so anyone feel free to correct me where I'm wrong.
So moral of the story, go bigger in the back if you're worried about messing up your awd..

 

YJosephTT, your thinking makes perfect sense. I think that the biggest issue here is that we don't know exactly how the awd system responds to "X" amount of variation in overall diameter front to rear on the cars (at least last time I checked I don't think we had a definitive document/statement from Porsche about this?). So we pretty much have to speculate like this.

My understanding (please, correct me if I'm wrong) is that our vicious coupling system relies on temperature of the internal fluid to strengthen (send more power to the front) or weaken the coupling. The temperature change of the fluid happens based on friction created by a difference in rotational speeds on either side of the coupling. Also, counter to what we would normally think... the hotter the fluid gets the thicker it gets and the stronger the coupling gets.

I think the more we deviate from the stock ratio front to rear the more slipping will be taking place which will heat up the fluid more. Stock overall tire diameter ratios front to rear are very close and actually slightly smaller in rear than front.

Optimum would probably be to keep this ratio as close to stock as possible. If you not only increase the ratio of overall diameter but also change it to having a larger diameter in the rear vs front the coupling will be "seeing" even more of a difference from what it is "supposed" to be seeing normally. This may or may not cause a problem depending on how sensitive the system is.

Keeping in mind that there has to be some lee way because tires wear unevenly and even different air pressures could slightly change overall diameters for example. My thoughts are that small changes in overall ratio front to rear should not be a problem but if you want to play it really safe just keep the stock ratio. ymmv of course

 

If you go bigger in the back the amount of torque that is transmitted to the front axle is reduced. If the viscous coupling discs driven by the transmission spin slower than the discs connected to the front diff then there is no torque transmitted to the front diff.

In fact in this situation then the faster spinning discs of the front diff are actually acting to brake the discs connected to the transmission.

As long as the speed difference is within the range with the proper sized rear and front tires the viscous coupling should not be harmed (no guarantees though) but any "advantage" of the AWD is lost and some performance is sacrificed.

My advice is to work the numbers of the factory sanctioned wheel/tire sets and come up with differences in tire rotation speeds between the rear and front tires. If one then wants to use non-sanctioned wheel/tire combos before purchasing work the numbers. If the differences in rear and front tire rotation speeds is within the range of the factory sanctioned numbers then the non-sanctioned wheel/tire combination is probably ok (but again no guarantees).

If one ventures outside of these numbers the viscous coupling or front diff probably won't blow up right away. Depending upon how the car is driven the difference in rotational speed may not be enough to cause the viscous coupling fluid to overheat or suffer whatever it suffers when tire roation speeds are too far out of spec.

So just because someone has fitted wheel/tire combo that has rear/tire rotation speeds outside of those factory sanctioned wheel/tire sets and has not had any problems is not sufficient evidence it is ok to ignore/violate the tire rotation speeds of the factory sanctioned wheel/tire sets.

The viscous coupling fluid has a working temperature of around 300F (or higher, I seem to recall it is in the area of 350F). That is damned hot. The only cooling this viscous coupling gets is from air flow along with how much heat it can shed to the fluid of the front diff in which the viscous coupling is located. Likewise the front diff is cooled only by air flow.

My info is the max torque -- 40% -- is transmitted to the front diff at approx. 150MPH. While that is a lot of torque -- the viscous coupling is amazing in that with just fluid -- albeit special fluid -- it can transmit that much torque -- that is a lot of air flow.

(As an aside for those that track their Turbo the above strongly suggests a cool down lap (or two) is really necessary to let the front diff -- among other things -- shed its considerable heat load. While one can let the engine idle as it sheds its heat load a stationary vehicle means no air flow past a superheated front diff and this exposure to elevated temperatures can possibly lead to premature failure of this item.)

'course, the heat transferred from the viscous coupling to the front diff fluid causes the front diff to run hotter which can lead to front diff bearing failure.

All I can offer is the advice to pick non-sanctioned wheel/tire combinations very carefully.

 

That is the question of the day,year, decade when it comes to tire fitment.
Where did this 3% or 4% differential ratio come from, so far it seems to be pure speculation. One thing I have observed while reading through posts is that the members who have reported PSM issues while doing track events have had close to stock rations or slightyly smaller rear diameters.
I really don't know how the viscous system works, but I would think that it would only activate in one direction for our type vehicles, unlike a subaru which can apply traction to any wheel at a time.