I was about to get a set of Pilot Sport Two's for my 996 tt, and was aware that the exact 225/40/18 front size was not N rated, but took another look today and noticed the it is reinforced as well (RF designation), while the 235/40 N (Porsche) rated isnt RF. There IS a 235/40 (non N) RF which has a 150 lb greater load rating and is 1/2 lb heavier than the 235/40 N (since I cannot use these sizes I am citing these for comparison between "N" and "RF") which tells me that there is a fundamental difference between the between "N" and "RF", definately sidewall stiffness and weight, possibly compound??.

I am now concerned enough that the current 225/40 RF available would probably not be the tire that porsche would approve for this car (it would be a non-RF version which does not exist yet) that I will probably stay with the N rated regular pilot sports unless someone talks me our of it.

Please comment!, thanks.

Bond

 

Just get 19" wheels and tires..

 

Seems like I read that the 225/40-18 was designed for a Volvo or Audi or something like that.

 

It's sized for the Chrysler Crossfire : Michelin PS2 specs

 

Thank you for bringing this issue up bond. I was going to order the correct size for the tt but now I know better. Is there an issue of running 235/40 sizes infront instead of 225/40??

 

I may be missing something here (hope not, I've orderd the PS2's and am told they will be here Monday) but the PS2 TT OEM is the N2, 235 front (it will fit stock wheel size), 295 rear. The Michelin guide shows it, and Porsche printed an OEM tire guide that confirms it...I hope.

 

In my calculations, the speed difference between the front and rear when using the next larger size in front (235/40) is too great and the AWD viscous coupling will overheat. The speed differential (all 4 new) from the 235/40 front is three times greater than if the 295/30 rear tires are bald and front standards (225/40) are at 50% which is typically the most extreme case. 225/40's are 829 rev's per mile with 295/30's at 832 and the oversize 235/40's at 819. I originally ordered PS2's too and decided not to risk it. (3 rev's per mile diff for stock vs. 13 rev's per mile w/235 - a greater than 4 fold difference - the AWD viscous coupling speer differential between frt and rear creates the normal 5-40% power transfer to frt wheels, at this (4x) speed diff, the front will never see 5% again, more like 20-25% all the time)

Bond

 

I have a 993TT and am worried about the same issue. However, the revs are closer

Stock
225/40/18 (827)
285/30/18 (837)

PS2
235/40/18 (819)
295/30/18 (832)

The difference is 10 revs vs 13 revs so it should be okay. Would be interested in hearing from others.

 

ok. First, i can't find anything that is 3 rev's difference. In fact, I get anywhere from 14 for OEM Bridgestone Potenza, to 25 rev differnnce if you go with OEM spec P Zeros (225/40 ZR18, 25" and 826 rpm(ile); 295 30 ZR18 25" and 801 rpm--can that be right??-see Tirerack.com for spec, Pirelli's site is no good). Also, is it true that an additional 4 wheel revolutions per mile are going to cause a 400% increase in power transfer to the front end?
I am new to this game...is it always this diffucult? I just want to buy 4 new tires? Anyone out there put this to rest?

 

"Also, is it true that an additional 4 wheel revolutions per mile are going to cause a 400% increase in power transfer to the front end? "

It is actually 4 times the differential in my example, not 4 more rev's per mile - I ended up getting the Pilot Sport Rib because teh Michelin tech at the US headquarters told me that the dry traction is basically the same between the PS1 rib and PS2, with the major improvement in wet traction, which I would not use (don't corner in the rain)

Bond