Great news that they fit and no more PSM light!!! Are you running the stock struts or do you have coilovers? I may try 225/40/19 295/30/19 next set, or go with your sizes depending on feedback...

Also, any chance you can post a pic of the rears from behind the vehicle? I can see what you mean about the rim being wider than the tire in the 3rd pic you posted, it's just hard to tell how much...

Finally, you have 11.5" rears, not 12", correct? Thanks for the post!

 

So there are a lot of numbers and opinions flying around in this thread... What was the 'final' on at 305 out back and a 245 up front? My new wheels will be here next week... 19x9 up front and 19x11.5 out back... Lowered with H&R sport springs... thoughts?

 

Read the thread below. It may help. There are soo many variables to consider. All the way from off-sets, to the cars height, to AWD, is it a 996 or a 997, the feel you want, do you care if you have to bend the front radiator bracket or not, and on and on and on... My personal preference was to get my tires as close as I could in diameter to limit drive-train slip, use the same tire brand on front and back and have a firm feel. My car is lowered, which allows the widest part of the tire to be above the front fender radiator bracket hump, which seems to be an issue with any tire that has a larger diameter than the 235 35's. The 315 25's would probably be the best match with those considering the revs per mile, but the tire selection is very limited. I think I got really lucky with the 245 35's on front. I selected those so the front would be close to rear diameters using 295 30's. My suspension and off-sets allowed those to fit by a hair. You also have to know the brand of tire and compare diameters. It would be best to use the same brand of tires if possible. My HRE 590R's off sets were determined by HRE for my car. (Look at Vivid's web page) I'm not sure what the exact off-set is because those rims were on my car when I bought it, but I'm sure if you spoke to the awesome folks at Vividracing, they could tell you. I have not found any perfect solution for ALL 996TT's. Here. Read this and decide for yourself.

https://www.6speedonline.com/forums/...ire-se-up.html

 

I run Champion RG5s 19X11 with H&R springs. I ordered Bridgestone Re050As 305/30s from TireRack last week and the clearance was too close (index finger horizontal would barely clear). Ended up ordering 295s and removing tires with less than 1 mile (expensive lesson). Not sure of the offset on the Champions as they were on the car when purchased. Tires will be going up for sale shortly

 

If your index finger fit then there were no clearance issues, no? The wheel size and offset is marked on the back of the wheel on one of the spokes, btw....

 

After reading all of this, I am less and less likely to go with 19's. 18's are less expensive and there are tons of tire options, especially in the 235/315 options.

 

Until you load the car on a corner

 

OEM Bridgestone 225/40/18 and 295/30/18 are 0.4" different in size according to the Tire Rack specifications. 25.3/24.9" and 823/837 revs. I guess you can assume that 0.4" difference in diameter is well within Porsche 996tt spec as a result since as the rears wear faster than the fronts the difference in diameter will only increase.

 

The 996TT OEM sizes were chosen that way (slightly larger fronts) to cause positive torque transfer to the (slightly slower spinning) front wheels at high speed through the viscous coupling. Obviously, this positive torque transfer also happens at low speed when the rear tires slip (begin spinning faster than the fronts) and the viscous coupling transfers torque to the slower spinning front drive.

Bottom line: A 0.4" taller front is NOT the same as 0.4" SHORTER front. The OEM tire sizes were designed that way on purpose. I will find the link that describes the AWD system and post it for reference.

If you have shorter fronts, the viscous coupling will be transferring NEGATIVE torque to the front at high speed! (Acting to slow the front wheels down!). Hence all of this discussion on what tire sizes/combinations both fit and limit this bad (unintended) torque transfer.

 

Here is a good description and explanation of the 996 AWD system from Streather's book (pages 257-259):

http://books.google.com/books?id=Raa...eather&f=false

Note that he admits that there is no official documentation regarding specific wheel size and resulting torque transfer. He just says stick to stock...

 

Were Bridgestones ever offered from the factory? I know Continental and Michellin were. The later are only .1" off.

 

This is a good read and thoroughly explains the workings of the front drive system with the viscous coupler. One thing is clear, since our cars have the same 3.44 final drive and front drive ratio, YOU MUST have a slightly smaller rear wheel than the front wheel in order for the system to transfer any torque to the front wheels. The smaller the rear wheel is than the front (greater delta) the more torque will be transferred the the front wheels in normal driving. If both your front and rear wheels were the exact same diameter than you would get zero torque to the front unless the rear wheels began spinning at which point a certain amount of power (up to 30%) would get transferred to the front wheels. If you HAVE EVEN A SLIGHTLY LARGER REAR WHEEL THAN THE FRONT, you will be transferring negative torque to the front wheels (the fronts will act like a brake) in normal driving and the only time you would transfer positive torque to the front wheels is if the rears lost traction / started slipping and exceeded the speed of the fronts. So, the most important aspect of our AWD system is that in order for it to function properly you can NEVER have a larger tire in the rear than in the front. It's not that you will burn up the viscous coupler but rather that you will be transferring negative torque to the front wheels. The only limitation that Porsche puts on the AWD system viscous coupler is the 5mph difference between the rotation of the front and rear wheels which works out to 21 revs per mile difference of the front to rear axle at 180mph. This limitation most likely has to do with the overheating of the temperature sensitive silicone fluid due to rotational differences of the input/output vanes in the viscous coupler.

 

How exactly do you transfer negative torque?

 

"Negative torque" = the viscous coupling acting to slow down the front wheels (essentially applying driveline braking)

 

Divide by zero?