Since I have PS2's with 315/25/19 in the rear and 235/35/19 up front, will 295/30/19's still be ok with my 235's up front. I just want to make sure my rolling diameters will still be in spec so abs/psm does not go crazy again.

 

Nope. The 315 diameter is 25.2, the 235 is 25.5 (close enough),
but the 295/30 is 26", which is way off.
Joe

 

use this:

http://www.miata.net/garage/tirecalc.html

I know...miata?

 

Hi folks. danger danger. No offense to wross996tt, but never,
never use any online 'tire size calculator' if you are dealing with
close tolerances, such as trying to prevent rubbing. They all
mean well, and are simply designed according to what the tire
size nomenclarure is *supposed* to mean, but in the real world
there is a wide range of variance between what a tire is
designated as and what it's real dimensions are. For the real
facts you have to go to the tire manufacturers spec sheet. I
have seen two different 245/45-16 tires differ in diameter by
1/2 inch *from the same manufacturer*! They were just different
model tires. Recently one tire manufacturer stopped making
it's 305/30-18. Now they list a 315/30-18. *It's the same tire*!
They just decided that the actual tire dimensions were closer to
the 315 meaning, so after 6 years they just changed what was
printed on the sidewall.

Joe Weinstein

 

so true Joe and good stuff! +1

 

Joe I absolutely agree, but the calculator will at least get you in the ball park ...or should I say eliminate the theoretically unreasonable sizes. OK?

 

For the most part, yes, but only if all you need is to be
in the ballpark, and in some cases real tires aren't in
the right ballpark. With the 996tt or any AWD car that
has some tight and explicit 'ballpark' requirements, I
would always go directly to the tire spec page such as
from tire rack. I have had a Porsche shop manager spend
hours debugging ABS issues and eventually trace it to
tire diameter changes due to the fronts wearing at a
different rate than the rears, so tires that were OK
enough originally started causing problems later.
As an example, I just looked at the published tire specs
for every 265/35-18 performance tire sold by TireRack.
The actual diameters varied from 25.0" (eg: Bridgestone
Potenza S-02 A) to 25.5" (Pirelli PZero Nero), and
even varied by .3" between different models of the same
manufacturer (Bridgestone Bridgestone Potenza S-02 is
25.3", but it's S-02 A is 25.0")
Bear in mind that besides any rubbing issues or ABS
failures, the power to the Porsche front wheels is via
a viscous differential, and if the fronts are turning
*any* differently than the rears, there's a constant
friction load on that differential any time the car is
rolling. It can take some, but I'd like to make sure I
minimize it.
Joe

 

Since it is not advised for me to use the 235's up front, do you think it may be ok since the fronts are somewhat worn down. They have about 8-9k miles on them.

 

no, that makes it even worse. The rear tires you suggest are
much taller than your fronts are new. If they are worn they
are even shorter.

 

Don't forget, our websight has manufacturer supplied specs for all of our tires. Some do vary quite a bit.

By the way, 235/315 is the best combo with AWD.

 

Haha...your right...what the hell was I thinking???
I guess I will have to put the new fronts on that I have that are 235/40/19..I think

 

As pointed out above, you will have a greater rolling diameter with the 295/30's
However, 295/30's will not cause an ABS / PSM fault.
I have used the 235/ 295/ combination on several 996TT's without issue.

If you want to see something a little extreme, this car

runs 245/30/20 & 305/25/20
Horribly over geared, which detracts from its acceleration, but still no ABS / PSM issues...

 

I think you said you wanted to run a 295/30-18 in the back with a 235 worn in the front. If that's the case, it might be ok.

But why not just stay with 315/30-18 in the back?

- KJ

 

So did I think, but changed to new tires now and got the ABS/PSM fault at about 140 km/h.
Anyone else got the same issue?

 

^ most everyone gets an occasional "abs/psm" when swapping to new tires, esp if mixing say "new" rears to old fronts, until the ecu adapts to the minor difference(s) in tread depth.

they ( psm cels ) will stop after the tires are properly scrubbed, absent a "real" issue.