Got a question for you guys. OEM tire sizes for my car 235-35-19 and 305-30-19 . Diameter of these is 25.5 " and 26.3 " respectively. Now given the obvious concern over rolling diameter of the tires on C4S and the impact of too much variation in front-to-back tire diameter on the drive train why not use a 235-40-19 on the front which at 26.4" and nearly identical to the rears 26.3 ? Tires in question are Michelin Pilot Super Sports.

 

Question is why the change but here's a comparator.
http://tire-size-conversion.com/tire-size-calculator/

 

INTERESTING, I see the difference is small and with the 235 40 19 is it almost equal. I would guess Porsche has already figured these numbers into their calculations as far as the 4 wheel drive is concerned. Subscribed.

 

2nd guess the Porsche engineers if you want but I'm sure they did the math before the cars ever left the factory.

 

My interest is more out of curiosity than anything else and unless I get a definitive answer from someone here on the board I wouldn't change.
But, I have read in the past that the diameter variance between the front and rear tires on my car needs to be within a certain small tolerance to insure you don't damage the AWD system over a period of time. This 235-40 size is nearly exactly the same diameter as the rear tires and therefore present a far smaller size variance that the 235-35 does. I can't image it looking out of place nor having any affect on the handling of the car and it's much closer to the ideal size from a diameter standpoint.
Could it be that the 235-40 is a newer size that was not available when the AWD system in the 07 cars was developed ? Got to be an answer.

 

996's use approx the same OD front to rear.

997's use an OD that is approx. .75" taller in the rear than in the front.

991's use an OD that is approx. .5" taller in the rear than in the front.

Don't vary that sizing difference any if at all possible.

 

The key is that they are not the same, you must maintain the same ratio. By using a larger front, you alter the ratio and will give yourself problems.

 

So you're saying that they are different because the gear ratios variances front to back require this variance in diameter of the tires to reduce/eliminate/minimize wear to the drivetrain ?

 

Dadio, have you tried to contact porsche directly to see if theres a problem using that size tire?

 

More or less. This is on 4WD cars like your 4S or a turbo. It is the center diff. The tires aren't spec'd to be the same size, there is a ratio between the front and the rear, which should be maintained in order for the center diff to operate properly. For example, running a 315/25-19 in the rear (perfectly acceptable for a 996 turbo) will not run well on a 997. I tried it and the car had some really weird issues. People go to 245/30-20 and 325/25-20 for 20" wheels (this keeps the front/rear ratio virtually unchanged), but very few make size changes on 19s. And even though a 245/325 combo is spec'd on a GT2RS, keep in mind that car is only RWD.

 

What ryem3 says - the issue has been discussed for a long time. Here's some more interesting reading although it is specific to rear tire size.
https://www.6speedonline.com/forums/...5-s-rears.html

 

Interesting topic..been scratching my head for weeks trying to make some sense of this. My recent purchase has large 20x13 rear and 20x10.5 fronts and I am in need of tires. I cannot seem to to find anywhere or anyone to give me the ideal setup for this if any. Sizing issues front to back and load rating for the rears. Love the current wheels but don't want to jeopardize the car long term.