Important - DO NOT put 315's in rears!!!!
 

Hi guys,

More to follow but my car is in the shop as I have an over heated front differential clutch. This is not the actuall clutch, but a clutch in the differential. (this showed up while looking for fault codes). It shows up on the computer as an overheated clutch.

Porsche is looking into this but the preliminary diagnosis is that it seems to be related to the height of the rear tire, in relation to front/rear tire size. The front tire height is the same as stock, the rears are an inch shorter, which seems to have burnt out the clutch in the front diff.. They are saying that difference can definitely burn the clutch and is not warrantable.

Therefore I am going back to stock tires sizes.

More to follow but you might want to hesitate on going with 315's at this moment.

Thanks!

Thank you for posting this Fred. Hope all works out the ebst for you on this issue. These new cars are very tempermental!

Sorry to hear about the troubles with your new car. Who on earth let you leave their shop with rear tires that were one inch shorter than the fronts :confused: Anyone above Walmart should know better than this :rolleyes:

Wow! And to think I was considering going with 315's for my next tire set. Thanks for the warning, Fred.

But if you match the front tire size, wouldnt 315's inthe back be ok?

<FONT color=black>

Sorry to hear about that Fred. We had a 997TT here in the shop a few weeks ago with 315's. Todd spent all day on the dyno pulling his hair out with the car. It was 100% stock and we wanted to get a baseline number on it before we upgraded him with our new EVT6XX system. The car had major power fluctuations throughout the <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-comhttps://www.6speedonline.com/forums/...e><SPAN style=tuning</SPAN></st1:PersonName><font color=" /><st1:stockticker><FONT color=black><st1:stockticker>RPM</st1:stockticker><FONT color=black></st1:stockticker> range and acted it very strange. We then thought that maybe the tires were causing the PTM to freak out so we installed the stock wheels and the problem went away.

<FONT color=black>The rolling diameter is different enough to cause the traction management to freak out. Also, we noticed that on the street the car would "hunt" and pull to the left and right under acceleration. This is a major problem as Fred has experienced first hand. Hopefully you can convince the dealer to take care of it for you. Keep us posted<O:p</O:p

<O:p</O:p

What do you think is a safe diameter difference? Is it the same on 996's?
Thanks.

As well, due to the tires in the rear being too large, I was getting the PTM warning on the dash. " PTM briefly inactive - too hot " . It appears and goes away all the time!

Boy these cars are sensitive, I hope it works out for you Fred. Another reason to leave mine stock when I get it.

That's why they recommend staying within 15% of stock spec. GL though

That is 4% no 15% - at least on the rolling circumference.

As a general rule, you want to keep your tire rotational speed identical to the stock speed, and in effect proportionally stock tire height for the front and rear wheels. If you don't, the difference in tire circumference and hence wheel speed will quickly start roasting diffs.

The width of the tire doesn't matter. The overall height of the tire does. You can run 315/25/19 rear tires just fine, but you will need to size the front ones appropriately. I believe the closest you could get with commonly available tire sizes in 19" would be a 265/30/19 in the front. It would still be slightly slower than the stock front wheel speed, but much less so than 315's and 235's.

Alternatively, you could just rip the AWD system out and run whatever you want. :p

Damn that is crazy Fred, sucks to hear :(

Fred sorry to hear that lets us know how things turn out!

How many miles did you log on your car with the 315's?