ajgordin

I totally understand where you are coming from OP. I, too, have looked into tire options in anticipation of when I have to replace mine. Unfortunately, almost all of the OEM-sized tires are N-rated options that are still made (bridgestone, pirelli, michelin PS2) and are very expensive ($13-1400/set iirc), and not as good as modern options such as the PS4S (they cost more too). Even the stock 19" 997 tires offer cheaper alternatives for good tires such as conti extreme sport contact or bridgestone s-04 (both run about $1,000 per set).

I'm hoping that Michelin starts offering the PS4S in our size, but dont see it being highly likely. I may either go to a 19" wheel or try out a set of the Sumitumo tires for less than half the price. A lot of guys on the turbo forums seem to like them as strath said.

charlieaf92

Porsche say to get it as close to even as possible, but definitely not to exceed 3% circumference difference. Many people on the forums have said that having front slightly larger than rear is the preference (vs. the rear being larger than front). I've found this to be true. The rear tires are typically inflated to a higher pressure, and increases their size - so I'm guessing that has a lot to do with it.

parris

I have mines mounted with 235/35 R19 (25,5 Diameter) on the front and 295/30 R19 (26 Diameter) on the back, four corners on Michelin Super Sport.

I think it should be a good setup, since it is the standard setup for the 997 C4S with 19"s, and the AWD system is basically the same (997.1 had the same viscous multiplate clutch).

I also have read several times that the rear diameter should be as close as the front as possible (or even smaller) but I dont know what is the basis for that, taking into consideration the 997 setup is like that.

An alternative could be, to mount on the rear 285/30 R19, which are closer to the front (25,7) and will fit on the 11" wide wheels.

GT3junkie

Thank you for your reply. I've decided to go the Sumitomo route until I figure out get new wheels. the PS4S are very well worth the price. They are in my opinion the Best all around premium tires on the market.

GT3junkie

Interesting... Thank you for the info. It helps a lot.

GT3junkie

Interesting info. Will look into that If I go the 19" route. Thank you!

Simon Guertin

I have the same issue, but for the C2. I have purchased the correct size tires for the front, but my rear tires are still good for another 2 years.

craig001

My C4S came with 225/40/18's one the front and 295/30/18's on the rear. I have 235/40/18's and 295/30/18 on a set od Kenisis rims and 225/35/19's with 305/30/19's on a set of 997 TT rims, For St Louis, the 19's gave me too harsh a ride with a crap roads.

parris

I have read that a lot of times, but as far as I know, there is not technical justification for that. Where did you get that information?
Actually, the 997.1, which shares the same AWD system than the 996, runs on wheels with bigger diameter on front. Besides, according to Adrian Streather statement in his book "Suprem Porsche", there is no documentation or technical reason for that requirement neither.

Just want to know if I am missing anything.

strathconaman

So, the 997.1 and 996 share the same front limited slip viscus coupling system. It works like a torque converter, in that power is transferred when the input shaft (running from the front of the transmission) is turning faster than the output shaft (leading to the front, open differential). The actual viscus clutch part number is the same between 997.1 and 996

In the 996 this means power is only transferred forward because the rear tires are smaller than the front, and that, coupled with the final drive ratio for the front sets the conditions for the input shaft to turn faster than the output. The 997.1 has a different final drive ratio for the front. So the rear tires can be larger than the front, but the input shaft still spins faster.

What I don't know is if the different front final drive ratio is built into the G97/31 gearbox or if it is in the front open differential on the 997.1.

parris

Thanks Strathconaman,

What you mention, is only valid for the "stationary" split of power between both axles. If you modify the circumference of the wheels, you are modifying the differential speed between both shafts. Even though, that effect is really low (in comparison with the effect of the final drive ratios) on the split of power between the rear and the front. We can be talking of having 4% in the front vs 5% in normal conditions. However, when the rear wheels spin faster because they slip, the 4WD system is going to work the same way.

I will tray to reproduce the part of the book that I mentioned where it is actually addressing exactly that aspect of the 4WD system.

strathconaman

Have a read of pages 258 and 259 of Mr. Streather's book.

BAD.TAG

Looking into some summer options and this thread has been a big help.
This site is great for comparing F/R options in relation to % difference for the AWD concern - https://tiresize.com/calculator/