While I can't find it now, I always thought it was 4% max. However, the manual DOES say that if the tread depth of a new tire is more than 30% more than the old one, that's a problem. I would have to imagine that if Porsche thinks tread depth is a cause for concern, tire sizes would be critical. Manual is 2100 pages...tired of looking right now...

 

All I know is that I spun all 4 wheels the other day in 3rd gear going up the freeway onramp and I stayed at a Holiday Inn the week before

 

I read 4% being porsche spec from some guy earlier on this thread...I did not verify that number...could be totally off.

Besides wear differences, which I totally agree with, let me throw 2 other variables out there that may effect functional diameter of tires:

Camber..increasing neg camber when we lower/modify suspension should increase diameter I think. Should change more for rears given width of tire.

Air pressure..ever drive on a partially flat tire without blowing up your diff?

While I will still try to maintain as close diameters as possible, I think 1 or 2 (or even 3) percent is more than reasonable.

Also, I have a hard time believing anyone can tell a .1 inch change in tire diameter (when your fronts go from .3 to .4).

 

You wouldn't feel it in any type of reasonable street driving. In the case of
taller front tires (the direction of difference I have been involved in), it can be
felt when I am racing, during hard trail braking. This may also involve/affect
PSM and ABS, which are always on during braking. It acts as if the car's front
brakes are fading, because the differential is still 'adding go' to the front, and
maybe also PSM/ABS is refusing to add as much front braking as I want
because the fronts are already not turning as fast as the rears. To the extent
that ABS compares front and rear wheel speeds, having different diameters
would generally diminish maximum straight-line braking.

Camber might have a little effect on the effective rolling diameter of the tire,
if it has a softer sidewall, but the effect would be to lower the effective
radius, not raise it. This is because it effectively puts more of the load on the
inner sidewall, rather than it being 50/50.

Pressure within a reasonable range might, a little, for weak sidewalled tires.
You can easily test this for your car and tires, with a ruler and tire gage,
wherever you can pump up your tires: Just measure the ride height (between
body and ground) near the rear tire. Then bleed off 5 lbs and measure again.
Some tires have very stiff sidewalls and won't show a difference, and some
(cheaper and/or those designed for more comfort, usually) are very flexible,
and might.

HTH,
Joe

 

I think that if you have people doing stuff like this and they don't blow their diff, it isn't as big a problem as some make it out to be.

http://www.youtube.com/watch?v=LUSYx...eature=related

http://www.youtube.com/watch?v=r3Pi5...eature=related

http://www.youtube.com/watch?v=Q84jf...eature=related

or maybe these were the cars that created the front diff scare

 

Im sure you are all getting bored with this thread... but just thought Id give you an update...

Tossed out my wrong sized techarts and but a set of BBS 19s 8.5 front and 12 back so I could put the correct size 19inch tyres on... 235/35 and 315/25 mich ps2s. Guys found the gearbox mount was broken so replaced that, changed the tip transmission fluid and aligned the wheels etc.

All the wierd graunching noises gone, diff has stopped whining and the car feels just like a car should with only 38000km on the clock should. And feels better than ever, with some decent lightweight wheels on...

It is totally unbelievable but thankfully true...

Can only assume with the old 235 295 combo that the front or back tyres just started to wear too much and put the differnce between the front and back too far off and the abs or psm started going nuts.. broken gearbox mount may have been the bigger problem though I guess.

Who knows, but am happy as a pig in mud, and good excuse for the BBS rims...

 

Love the rims. Very "stock" looking

 

Reviving an old thread here...

My presumption to this point is that we are to be concerned in the OD variance between the front and the back. The point isn't to keep the same OD from OEM, just the ratio.

With that said, I selected the following:

19x8.5 wheel - 245/35/19 Michelin PSS - OD 25.8"
19x11.0 wheel - 295/30/19 Michelin PSS - OD 26.0"

Variance front to rear = 0.84%.

I think I will be perfectly fine at this variance.

Any thoughts?

Chris

 

From my experience 2 posts above I wouldnt risk it. Having said that In hindsight my problems I suspect may have had more to do to with the broken gearbox mount rather than the Tyres. But for me, if you can get the right size tyres (and I know they can be a bit tough to find) then why would you go for anything else ? Plus 295 is IMO a bit small from a looks perspective, when 315 fits perfectly. Just my 2 cents worth

 

I'm pretty sure those will rub. The goal is too keep the diameter as close as possible front to rear. I think the variance you have is fine, but I don't think they will fit a 996.

 

[quote=Hams996TT;3287941]Reviving an old thread here...

My presumption to this point is that we are to be concerned in the OD variance between the front and the back. The point isn't to keep the same OD from OEM, just the ratio.

With that said, I selected the following:

19x8.5 wheel - 245/35/19 Michelin PSS - OD 25.8"
19x11.0 wheel - 295/30/19 Michelin PSS - OD 26.0"

Variance front to rear = 0.84%.

I think I will be perfectly fine at this variance.

Any thoughts?
HI,Contrary to all these theories on here...I speak from experience.. I have been running a .5 inch difference for over 22,000 miles,numerous dyno runs,many 180 mph blasts,2,000 mile road trips WITH no problems...and my Porsche techs say I do not have to worry with in a .5 range...25.9 rears and 25.4 fronts..Usually their is more to it when the dif. coupler goes out....This is my findings that I am passing on....Hope this helps....

 

I have been reading through this thread and still dont seem to be 100% on this issue. I just recently purchased a car and had it delivered to my home. The car has 235/35zr19 & 295/307r19 Pirelli's. If I have read correctly my front diff will go, am I reading the differences correctly? I should go with a bigger tire such as 315/30 correct? Sorry for the questions... The problem is here I dont have access to certain sites including tirerack.


Any help would be appreciated!

Oh this is on a 2003 996t



Thanks
Scott

 

Who knows... But for what it's worth run with 235/35 and 315/25 19s and you can sleep soundly.. My 2 cents worth...

 

Hi Scott,

Based on those sizes, you have a mismatch in OD that could lead to a failure. Therefore, you need to select a new tire for either the front or the rear to get back into spec. Since you have a Pirelli Tire; theoretically you could run a 245/35/19. The OD of the Pirelli 295/30/19 is 26.1"; the OD of the Pirelli N1 Rosso 245/35/19 is 26.0". That will give you the proper OD matching.

HOWEVER... I cannot guaranty that running a 26" tire on the front will clear the front struts. If you are running OEM hardware on the front, I am fairly confident it will not clear. If you are running a coilover, it is likely that you can run a 245/35 no problem with clearance. It will likely rub the inner fender liner and turn lock though, depending on the ride height.

So... The cheaper way would be to replace the fronts with a 245/35/19 Pirelli, but IMO it would be a gamble on your car depending on your suspension setup. I wouldn't attempt it unless you have coilovers.

The other, more expensive option: purchase a 315/25/19 tire for the rear. Unfortunately, Pirelli does not offer that size. Therefore, you willl either need to purchase a complete set of tires, or run a mismatched set.

So...

Choice 1: purchase 245/35/19 (likely wont work with oem suspension)

Choice 2: purchase 315/25/19 rear tire (and possibly new set)


Hate to be the bearer of bad news on that.

Best of luck,

Chris

 

i am now running a stock size 225/40/18 frt on slightly worn ps2's, and new 315/30/18 rears. since installing the new rears the abs/psm warning lights have thrown, but only once and have not yet returned after cycling ignition/overnight etc.

my question: does anyone have any experience with running the wider rear 315/30 with ( or without? ) issues. i realize i'd be better of upping the fronts to 235/40/18 but would like to at least wear out the existing tires without wearing out the front diff!

i've been told by more than a couple very knowledgeable people that i'm ok and within "spec" to run the sizes i am, but since i've gotten the dreaded psm/abs failure CEL, i'm really like to know if anyone has direct experience with this sizing issue. TIA.