I'd like to understand this too. Seems to invalidate the argument about needing larger front

 

What are the differences in diameter? I don't have the manual by me

 

Went ahead and checked the manual. Depending on the make and model of tire, it seems that the rear will be .1" or .2" greater than the front. My guess is that the difference is too little to seriously harm the differential??? Or Porsche themselves have messed up and the world is all going to hell (kidding)

 

My winter set has a rear tire .2 inches taller per the manual and my summer set is nearly identical (235/40f, 315/30r). I've ran these set ups for the last 20,000 miles with no issues or warning lights. No need to have a larger front at all. I'm fairly confident Porsche knows what they are doing.

 

I just believe that if the difference, with the rear being larger than the front, is very minimal, like .2", it will have almost no effect on this sensitive differential that everyone is worried about. In my case, there is a whopping .8" difference where the rear is bigger than the front, so I'm not comfortable with that setup. People just try to replicate the stock setup in which the front is .1" greater than the rear. In the manual, Porsche recognizes that many owners will stray from a stock setup, obviously. Therefore I would think in the warning section for tires, they would put in bold font MAKE SURE THE FRONT IS SLIGHTLY BIGGER THAN THE REAR if it was that crucial. As long as the differences in diameters are kept to a minimum, as an AWD vehicle should have, we should all be safe.

 

Please explain in layman terms how the 996tt AWD system will transfer torque to the front with a larger rear tire rotating slower than the front when the final drive ratio is 3.44 both in the front and rear. I'm not arguing, just would love to know....

 

Its German Magic Like Everything Else German

 

It's right there in black and white though, this isn't an opinion it's a fact. The manual recommends a larger rear for winter setup

 

Hi Guys....Zookie brought this thread to me attention yesterday so I thought I'd chime in and clear the air.

So as it turns out, there was in fact an error made by one of the sales guys here at our shop when recommending the 305/30/19 rear tire. Lately because of the shortage or discontinuation of the 315/25/19 PS2's, we've been steering many of our 996 customers towards the 305/30 rear, BUT we've been pairing it with a 235/40 in the front rather then a 235/35. Our sales guy made an error and got the front sizes wrong.

Using the 235/35 and 305/30 combination will definitely cause some issues with the AWD in the 996. Whether signs of those problems happen immediately or over prolonged use was a topic of a little debate when I asked some of our techs around the shop. Some said you would hear the diff right away, some said it would take years to show signs of damage. Either way, it's not optimal.

 

Well I just received my wheels from you guys at my house a few days ago. Despite how your salesman reassured me that it was perfect fitment, I was reluctant to mount them on to my 996tt. When can I send the wheels back so proper tires can be fitted? Also, my concern with the 235/40 front is that it's 26.4" and might be a little tight in the wheel well because of the bubble from the radiator bracket. This issue was brought up earlier in the thread. Thoughts?

 

Jay.... please contact the salesman you dealt with. He'll take care of you for sure.

As for the sizing...a lot depends on the individual car and how low it is. I don't have one here to check in order to give you a 100% answer on that. The worst case scenario there would be to move the bracket as mentioned earlier.

 

Thanks Tom I'll get on it

 

DAME!! Thanks Tom for the Input but a 235/35 PS2 is already a Tight Fit the way my car sits and is already Rubbing Against the Fender Liner & Bracket... 235/40 might be just Killer lols

How about 305/30 Paired with 225/40? This looks like a better Closer Fit than 235/40

 

Wish I could. I'm just saying what Porsche has in the manual. I drive my car year round near Lake Tahoe and we get a huge amount of snow. My AWD system works great with that same recommended tire setup. I know power is going up front as it is obvious when I hammer it in the snow (wheel spin in the front). So while I can't give you a detailed explanation, I can guarentee you it works or I would not be making it home in the snow as I have a 1 mile hill that climbs at 10%+ on my way home.

 

Why couldn't it transfer power forward? If the rear wheels are losing traction they will rotate faster than the front assuming the fronts aren't spinning? That .2" diameter difference probably makes no difference when you are considering how quickly a tire can spin on ice and snow in a given moment.