Jump to content

Server Lease Renewal/Software Licenses

Our yearly server lease, software licenses, as well as hardware operating costs, ARE due Dec 6th, 2021. Our current donations have fallen far short of the funds we need to renew. Please remember the RennTech.org community is Member supported so please consider a donation to help...  THANK YOU!

Welcome to RennTech.org Community, Guest

There are many great features available to you once you register at RennTech.org
You are free to view posts here, but you must log in to reply to existing posts, or to start your own new topic. Like most online communities, there are costs involved to maintain a site like this - so we encourage our members to donate. All donations go to the costs operating and maintaining this site. We prefer that guests take part in our community and we offer a lot in return to those willing to join our corner of the Porsche world. This site is 99 percent member supported (less than 1 percent comes from advertising) - so please consider an annual donation to keep this site running.

Here are some of the features available - once you register at RennTech.org

  • View Classified Ads
  • DIY Tutorials
  • Porsche TSB Listings (limited)
  • VIN Decoder
  • Special Offers
  • OBD II P-Codes
  • Paint Codes
  • Registry
  • Videos System
  • View Reviews
  • and get rid of this welcome message

It takes just a few minutes to register, and it's FREE

Contributing Members also get these additional benefits:
(you become a Contributing Member by donating money to the operation of this site)

  • No ads - advertisements are removed
  • Access the Contributors Only Forum
  • Contributing Members Only Downloads
  • Send attachments with PMs
  • All image/file storage limits are substantially increased for all Contributing Members
  • Option Codes Lookup
  • VIN Option Lookups (limited)

Can i use 285/35/18 for rear and 225/40/18 in front w/o effecting PSM on 04 996tt


Recommended Posts

Hi to All. I was wondering if i can change the rear tire size from 295/30/18 to 285/35/18 and keep the front stock as 225/40/18?  Would it possibly effect PSM getting activated in turns in road?

Link to comment
Share on other sites

There are number of tire size calculators available on the web.  These give theoretical comparisons and ratios.  It is best to measure the actually rolling diameter of each wheel/tire as inflated.  Porsche recommends less than 2% difference front to back.  Consensus is to have the front tires the larger of the two.

Link to comment
Share on other sites

  • 2 weeks later...

Short version: not advisable.

Long version: I have that exact combo (Continental Sportcontact2 225/45R18 & 285/35R18) on my 99 Carrera (cable throttle and TC only) since I bought it.

When coasting off-throttle from 50-60mph the bigger rear tyre circumference results in lower rotational speeds compared to the front wheels which trips the traction control into preventing engine braking by opening the iacv (and adding fuel & advancing the ignition most probably) as the difference in wheel speeds front to rear is normally indicative of impending rear axle instability  (oversteer).

Only when I tap the brake pedal (which closes the brake light switch) the TC intervention stops and engine braking is normal.

Pressing the clutch before tapping the brake pedal results in an idle speed of about 2100-2500 rpm.

Disconnecting the iacv results in pops and bangs on decel which stops also as soon as the brake pedal is tapped.


First I thought it was a sticking Iacv that wouldn't close to the appropriate idle position when releasing the throttle pedal from something close to WOT (BOSCH motronic 1.1 from the 80s opens up the  iacv at WOT that's what made me think 5.2 'does that too) but the exhaust crackle on decel after disconnecting the iacv when the car was idling normally indicated that something else had to be going on.

It was this thread in fact the post by flatsix36 that gave the actual cause of my symptoms:

PSM/ESP equipped vehicles will respond similarly.



Edited by hardtailer
Link to comment
Share on other sites

  • 4 weeks later...

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.