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

[X50]

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?

[wross996TT]

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.

[hardtailer]

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 June 9, 2019 by hardtailer

[X50]

Thanks i decided to stay with original size w/c is 295/30/18