Yep, perfectly safe for the AWD

 

Paired with what front tire size? The problem is never with the size of one axle, but with the front rear combo for awd.

 

Hey Damon,

Since we are running low on tire choices. What are the other preferred options for the people daring to live on the edge of close fitment? Any of these options better then the other?


(F)235/40 or (F)225/40 -- (R)305/30


(F)245/35 -- (R)295/30


(F)245/35 -- (R)305/30

 

I was recommended 235/35/19 with the rears being 305/30/19 by Champion MS. I have full faith in them

 

Any other confirmations before i make the switch?

 

Please don't take this the wrong way as I mean no disrespect but obviously you have no idea how our very basic purely mechanical viscous clutch AWD system works. Champion specializes in 997s but they should know better. The sizes they quoted you are for a 997tt which uses a totally different electronic AWD system. The 996tt relies on a specific tire size ratio front to rear to function properly. Do as you wish however.....It is your car...

 

I respect what you said. What do you think is the Option?

 

It's common sense that their needs to be a specific ratio of front wheel size to rear wheel size. This ratio applies to all AWD cars. But again, Champion assured me these tire measurements work fine for a 996tt, a few clients already have these same tire sizes; I made sure of it. Thank you for the concern though, I will definitely double check with others nonetheless.

 

A 235/35/19 (25.5"tall) and a 305/30/19 (26.3"tall) is not the option as the rear is 0.8" taller than the front. I am not sure why Champion is recommending this 997 combo. Ask whoever is telling you to use this to explain how the AWD system works on a 996tt and I suspect you will get a blank stare. Ask them to explain to you how the AWD 996tt system with identical final drive ratios could transfer torque to the front with a taller rear tire.

I just recently put Michelin 235/35/19 (25.5"tall) and 315/25/19 (25.2"tall) PS2s on my car which has 9/12" wheels. In my case I can't put anything narrower than a 305 in the rear. If I were to put a set of new tires on a AWD 996tt with 19s I would put the new Michelin Pilot Super Sports in a 305/30/19 (26.3"tall) and pair it with a 235/40/19 (26.4"tall) that way I was running a front tire that is just a tiny bit taller that the rear. As long as the rear is rotating just a tiny hair faster than the front, this assures that the viscous coupler is constantly transferring a small amount of static torque to the front. Remember, our cars use identical 3.44 final drive ratios both front and rear so we are relying purely on a difference in the rate of rotation front to rear to transfer torque. That's why Porsche will tell you that the faster you go the more torque is transferred to the front up to approximately 40%. This makes perfect sense because the faster you go the greater the difference between the rate of rotation between the front and rear. As the rears are rotating faster they are driving the fronts through the hardened silicone fluid in the viscous coupler. Most people can't seem to wrap their heads around this concept as they don't understand how a viscous coupler functions.

The 235/40/19 and 305/30/19 combo will work but your clearances will be very tight, especially in the front at the forward fender liner where you have the "bubble" for the radiator bracket. Proper wheel offset will be key or you may have to mod the bracket a little bit which is not that hard to do and was describe by someone on this forum before. A 225/40/19 (26.1"tall) and a 295/30/19 (26.0"tall) would give you a bit more clearance while still giving you the proper ratio front to rear. Hope this helps

 

check out the thread about front diffs being chewed up. I absolutely will vouch the 997 sizes are not a good choice.

 

stock 997 sizes have the rear being .5" inch taller then the front with that being worked into the design of the 997 AWD. 997 AWD is compeltely different and superiro to the 996.

997TT tire sizes will definetly chew up your front diff. so the 305/30 is too tall

245/35/19 and 295/30/19 will work fine.
225/40/19 and 295/30/19 as well as 305/30/19 will work fine too.

 

Well, I'm most certainly not rushing to mount the wheels anytime soon. The stock wheels with tires have a front height of 25.1" and 25.0" for the rear, so I understand your personal recommendation of sizes. Using the Tire Rack calculator though, I'm getting under 3% mismatch, which from what I've heard is perfectly fine. The local Porsche mechanic told me that what sizes I have now are fine, but I'm still questioning how the rear is still larger AND the difference in diameters are .8". Waiting for 315 PS2's is clearly not an option so what would you recommend with the sizes I have now?

 

So right now I have 235/35/19 for the front and 305/30/19 for the rear. What would you recommend I do, sell the rear set and get another pair? What sizes?

 

[quote=JayAndreTT;3575028]The local Porsche mechanic told me that what sizes I have now are fine, QUOTE]

Will he pay for your front diff when it fails or will you just go RWD ?

I would start by conacting the people that sold you tire sizes that do not work on your car. There are a ton of thread on this. Ona 996tt you want the heights to be as close to identical as possible.

 

[quote=prodigymb;3575032]They are pretty stubborn in that they claim the fitment is fine. Guess I should return them and just get a new set despite what they say?