I currently have this set up on my car and am concerned with the issue of having the front differential getting chewed up b/c of the diameter difference. Although, I've read a few people not having a problem with it. If you have ran this set up, can you please answer:

1) How long have you ran the set-up?
2) Any issues with the front differential?
3) What brand tires did you use?

Thanks everyone

edwin

 

I have that front tire size but a 315/25/19 rear no problems at all...I think that rear tire size your asking about is to tall for the front size tire.. think you need 245 series for the front

 

I run 235/35-19 front 305/30-19 Invo's. Was told by my tire rep that I use for customers cars, that these tire were within spec. Love the Invo's!

 

How long have you had them on? Based on Nittos website, the diameters for the 235/35 is 25.5" while the 305/30/19 is 26.2", which is a pretty big difference

Edwin

 

I have had them on for about a month. My tire tech said they had to be less then 5% different front to back (they are under 3%). So far nothing funny with diff. My wheels are from Forgiato. I am a distributor for them, and they told me, these are the sizes they have run on the 996tts since they started making wheels for them.

 

Hey Edwin... after three pages of rehashing the same thing on the other thread, I grew tired. (pun intended)

But think of this:

1. Stock diamtmer mismatch is 0.1"
2. Up to 0.3" is generally considered acceptable.
3. Your tires are 0.5" as I recall.

(Actually revs per mile is the best number to use- stock is 829/832 and your sizes are 820/836.... so 3 revs per mile mismatch versus 16 revs...or FIVE times the mismatch)

I suspect this is not a well defined issue. One might do fine with how you drive and your particular diff.... or it might fail.


What kills me is that elsewhere Porsche requires that if one tire is replaced on a AWD, they must replace all 4.- due to the tread depth causing rotational mismatch..... and here people throw around half inch mismatches and assert 'it works fine for me'...

TwnSpn- will forgiato pay for any blown diffs if you follow their recommendations?

 

I used to run this set-up back in 2004 on my old '03 turbo. Kept it for a year and I even tracked it twice. Tires were Contis and I guess I got lucky because I've never had a problem with the front diff.

Like I said, I got lucky.

 

If you're really worried about it, just remove the front diff and driveshaft for instant RWD.

 

To be accurate it's not the front diff that will be damaged. It's the viscous coupling. Basically a wet clutch that oprates based on speed differential. When the rear tires slip, power goes to the front wheels. Otherwise, the road keeps all tires spinning at the same speed and no "slip".

You can see how having different diameters front and rear would burn the viscous coupling by making it slip all the time.

 

Different platform, but 3000GT VR-4s also use VCUs, and a different diameter tire IS a problem. Maybe it will not blow your VCU right away, but it will put a perm. damage on it, so if not you, the next owner will run into problems eventually.

 

At dealer today and saw a new 997 Turbo with 235/35-19f 305/30-19r stagger from the factory. Don't they have the same V/C?

 

Sorry for bumping, but someone else might read this thread and get the wrong idea. With tires like the Michelin Pilot Super Sport, 235/35-19 needs 817 revs per mile, 295/30-19 needs 799. The difference is 2,25%. Then the rears wear down (they will wear out before the fronts) the difference will be about 0,37%.

The stock tires are as you say 829 front, 832 rear. -0,36% difference. When the rears are worn they will end up at around 847, or -2,13%.

All within spec which is +/- 3%.

 

After reading what feels like a million posts with almost as many differing opinions on the matter, I went with 245/35 R19 up front and 305/30 R19 in the rear.

The math looks right (click here for details), and stands to get only better as the rears shed a bit as Thorstein notes above.

Now... just need to get it back on the road!
-V