Does anyone know the maximum official factory F/R tire OD difference? I think I read it was 4% but I'd like someone to confirm that. Is there a way to get tech info like this directly from Porsche without going through the dealer?

 

Hi. I highly doubt that a 4% difference in front-rear tire
diameters is within tolerance. I don't recall ever seeing
Porsche publish a specification on this. You can infer an
acceptible range by reading the specs of the OEM tires
that Porsche suppliies with the car. The difference in
diameter of the Pirellis is closer to .4%.
Make sure you judge tires by their detailed specs, not
just the 225/40/18 numbers, because those latter ones
are *approximate*. I have seen two tires, both spec'ed
as 245/45/16 be over a quarter inch different in diameter,
and these tires were both made by the same manufacturer!
They were just different tire types (one was an all-season,
and one was more for dry).
Joe

 

I understand officially it is 1%. I understand people have successfully run close to 3%.

 

If you use the tire size spreadsheet you get diameters:

225 / 40 R 18 25.087
285 / 30 R 18 24.732

that's a front to rear difference of 0.014 or 1.4% from the factory. I haven't gone out and measured, but that should be close enough.

I'm putting 19" wheels on and am making sure I'm within that tolerance. I think with 4 wheel drive, front to rear ratios might be pretty important

 

Joe,
I agree.

cjv,
Where did you come upon this info?

Ken,
You are doing what Joe said not to do. There are inaccuracies in your method of comparison.

The bottom line is that the difference in rotational velocities of the front and rear wheels should not be more than those of the stock configuration. This is certainly the most conservative approach. Since the rotational velocity is directly proportional to tire circumference which is also directly proportional to wheel diameter (C=Pi x d), the ratio of front to rear factory tire diameters should be maintained. The best way to make the comparison is to obtain the specific tire diameters from the manufacturer of the factory tires and determine the ratio of diameters front/rear. Make sure your new setup does not violate this ratio. In actuality, unless I am overlooking something, the viscous coupling cannot tell forward from backward relative motion and therefore if the front/rear or rear/front ratio is comparable to the factory tire specs you should be OK. The enemy is heat. The bigger the difference in tire diameters, and hence rotational velocities, the more "slip" has to occur in the VC. The more slip, the more heat. The more heat, the more potential for wear and damage.

Of course all of this assumes you are operating at correct tire pressures. If you are running substantially lower pressures, the "apparent" diameter will be smaller.

Incidentally, Yokohama AVS Sports have the following actual diameters:
235/35/19=646mm (compared to the spreadsheet value of 647.1)
315/25/19-640mm (compared to the spreadsheet value of 640.1)

 

Audi used to beneficially play with front-rear diameter
differences to get different power balance, when they
raced a lot of AWD. Note that this *does* stress the
front-to-rear differential.
To clarify: A tire manufacturer provides *two* numerical
specifications for it's tire. The first is the standard form on
the tire wall "285 / 30 R 18" etc. THIS IS APPROXIMATE.
The second spec is the spec sheet, which TireRack or your
dealer should have. This will typically include a count of
revolutions per mile. THIS IS THE MOST ACCURATE measure
of actual rolling diameter.
Joe

 

Joe,

 

The information regarding the 1% was obtained from a dealer. He refered me to the two acceptable tires used by Porsche. They fall in the 1% range. The other percent is derived from what I have seen others successfully try without encountering any problems over some extended driving.

The upper figure I believe was also given by Viken on the Rennlist form. He did have some additional back up. You might ask him this question and the reference for his information.

 

This brings up an interesting question: When/if the front-to-rear
viscous differential fails, what is the mode or modes of failure, and
the symptoms?
For instance, if the unit merely stops transferring any power to/
from the front wheels, it might not be detected by a driver until
AWD was necessary.

 

let's make this even simpler. If I stack all four wheels/tires in one pile and they're all the same size, then I'm ok???

 

Does anyone know how to get a definite answer from Porsche?

 

Ken,

If all the tires were the same size you would be OK from the rolling point of view.

 

Ok - now I am really confused...looking at the following link http://www.conti-online.com/generato...tasheet_en.pdf
what fronts would be within the recommended tolerance to go with the 295/30 19 rears?? (i think mine are off!!) what about for the 315/25 19 rears??
thanx

 

The 235/35/19s would match the 295s fairly closely
at 808 rpm vs. 805, and the 225/35/19s would match
the 315s exactly at 825 rpm.

 

There are two 235/35 thogh...how do I know which ones I have??
Can you do the 819 235s with the 315?? I dont think 225 would look good IMO!