cayenne has 305-30-22 rear and 285-35-22 front can this setup damage gearbox on a automatic or what should the front be to be the same dia thx

 

285-35-22 = 29.9" Diameter, 93.7" Circumfrence, Revs/Mile 696
305-30-22 = 29.2" Diameter, 91.7" Circumfrence, Revs/Mile 712

(Reference: https://tiresize.com/comparison/

Porsche allows 30% difference in tread depth. Assuming a beginning tread depth of 10/32" (8mm) - the allowable difference could be 2.4mm (3/32" or 0.0937")

This would also be the allowable difference in radius. So..

Difference in diameter is 0.7", meaning the difference in radius = 1/2 that - or 0.35" - or 9mm.

The difference in radius of the two tires exceeds Porsche's allowable difference by a considerable amount. It would likely cause damage to the transfer case, and provide crappy handling. The difference would be the same as having brand new tires on one end of the vehicle and totally worn out tires - bare of any tread - on the other end - if the tires were the same size. That is not a good thing at all.

Looks aren't everything..

 

tolerance is 3% on diameter so
difference in your size is:
285/35r22 758,3mm
305/30r22 741,8mm

difference is 2% which is ok
even better would be to have rear 315/30r22 . here is 1% difference

 

758,3mm - 741,8mm = 16.5mm. 16.5mm/2 = 8.25mm. Where does the 2% come from? That's as I said - like having bald tires on one end and brand new tires on the other end = transfer case issues.

 

​​​​​​​if you take 758,3mm as a 100% then 741,8mm is 98% of 758,3mm, thats 2% difference

 

Where did you get the 3% difference allowable number from? Is that in a Porsche owners manual or service manual?

 

its not written in manuals.
I got info from tuners who make extreme stylings to this kind of suv.
Most common extreme is front 295/30r22 and rear 335/25r22.
Im also developing now my own brand of forged wheels so i make all possible variant of stagerred sizes of tyres
You must also know that there is difference at front tires and rear because tire pressure. This kind of sizes has front app.2.1bar and rear 2.3 bars, even here is difference in high.
Yours 30% probable means difference in roundness, the distance of path which tire do at one turn.
Im sure that yours 30% difference means somehow 3% difference at diameter.
Someone should calculate this