Can I run a .5 inch larger tire diameter in the Rear w/PSM?

I want to run a slightly taller tire in the rear. Its .6 inches bigger in diameter. Wonder if Porsche Stability Mgmt will have a fit? Appreciate thoughts but really would value someone who knows or has done it with PSM. thanks gents.

what are your f/r tire spec?

I have a 2000 C4, so stock PSM. The original tire sizes for 18" wheels were 225/40-18 on the front and 265/35-18 on the rear. I have been running 235/40-18 on the front and 265/40-18 on the rear, Michelin PS2's for three years now without problems. You can look up the diameter and the revs per mile on tirerack and compare the stock differential front to rear with what you are thinking of running and see how big a difference there is.

http://www.rimsntires.com/rt_specs.jsp here is the best tool I have found for comparing - at the bottom it give detailed info as to what the size changes mean Doug, you went to taller profile?

I have a '01 TT and would like to run 305/30 x 19 (26.3") rears and 225/40 x 19 frts (26.1"). And a second set of track tires 305/30 x 19 rear (26.3") and 235/35 x 19 frt (25.5"). Second question is do I have clearance with a 26.1' diameter frt tire, rears will be no problem. thanks

Doug, checked Tire Rack and your frts are 25.4" and rears are 26.3" thats a .9" differential. If no PSM issues I should be OK if I can squeeze my fat 26.1 in the Frt without tearing out the fender liner.

I went to the taller profile, which are 997 tires because I found a good deal on a new set. The rears will need replacing soon and I am considering going to a 285/30-18, or maybe back to a 265/35-18. I do have rolled fenders, did this with another set of wheels I used to use that had a lower offset on the rear. In reviewing the tirerack specs I am not sure how accurate they are. I see 225/40-18 Pzeros that have the same diameter but show revs per mile of 829 and 801, 265/35-18 that show 820 and 792. Maybe there are some errors on their tables.

To avoid ABS, TC and PSM issues (mostly ABS and TC by the way) the rolling diameter of the wheels and tires must be within 3 percent max front to rear. So you can use a taller front tire if you also use a taller rear tire. ABS TC etc. use the difference in rotation speed of the wheels to work their magic. If you have more than a 3 percent difference, then TC gets upset and ABS may go into "ice-mode" which is a bad thing on a race track or even on the road if you need to brake hard.

Problem, checked the clearance with my frt Struts and there is only about a 1/4" between the bottom of the strut springs and the frt tires. If I go from 25.5" in diameter to a 26.1" it may not fit. When taking into consideration that the frts are 50% worn, my clearance might be even less. Bob, the % differential is within the 3% variance so I should be ok and not disrupt the PSM, ABS, and TC.

Since your ride is a 4 wheel drive make sure to go up in size with the fronts as well as your rear tires !!! If you don't... you'll kill your viscous 4x4 coupling GUARANTIED. Let us know how it works out. Cheers, Sid.

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jeeze yah the fancy computer doesn't like mismatched numbers coming in. Even ABS goes off dealing with a wrong set up.

Sid & pac996, what do you base your comments as Doug is running a .35% differential from back to front and is not experiencing any problems. Doug how long have you been running this set up (265/40-18 on the rear & 235/40-18 on the front). Doug have you experienced any issues in running a larger diameter rear tire?

the rule of thumb is no larger than 4% delta f/r.... these cars are were designed with 25" tires in mind. I would rather keep the delta as low as possible.

Ferdinand, early Bob posted a 3% differential, is 4% delta different? If same who's right and what was the source of your numbers. If I go with a 25.5 dia frt and a 26.1 rear that would be 25.5/26.1 or 2.3% difference.

Based on research. Check here at RL and on 6speed oh and don't forget your local P. dealer. Cheers.

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Bob Reiner ? (watercooled 996/997 tech guru).... on the pca website has touted the 4% rule several times. Porsche never released that number, but he is supposed to be the expert... do a search on tire sizes on the PCA website, you will find that 4% number there somewhere.... Your numbers should be fine.... is it a c4?.... if it is, the threshold of the PSM will be reduced. (kick in earlier). It should not harm anything though. Going with 19"s will even be worse. As I said before, going closer to 25" would be ideal with the smallest amount of delta...