as stated, i found out the hard way, 235/35/19 is a NO GO with 305/30/19 in the rear

 

Define "the hard way" for me? Did you blow your diff with those or are you doing research here and just had to sell them and cost you funds for new tires?

 

I just ordered both the 18s and 19s and here is what I went with (hopefully I got this right)

235/40/18 fronts (Nitto NT-01) 25.39"
315/30/18 rears (Nitto NT-01) 25.35"

245/40/19 fronts (Nitto Invos) 26.73"
305/30/19 rears (Nitto Invos) 26.22"

 

Just took the time to read the entire thread, very informative.

Thank you for the effort!

 

Funny, I spoke to somebody about this issue who works on, races, and is a major Porsche person. He said this is internet lore, and it happened to one or two guys and everybody hangs on to it like it's the gospel.

Not trying to debate the merits of the matched tires, I know Porsche did it for a reason, I get that. What I'm interested in is, in the last 4 years I could only find in all the threads 1 or 2 anecdotal accounts where people surmised the differential in tires was the issue that blew up their front diff. Viscous couplers fail on all kinds of other cars (FWD, RWD etc) all the time where tire size is not relevant, they just die eventually not sure how Porsche is any different. I also know there are A LOT of 996TT with miss matched sizes, look at your next event.

With respect to sizing, I'm curious. The rule here is to say at the same rolling diameter yet there are so many variables. Tire wear can account for .75" on a new vs almost bald tire, that's nearly 3% there. Tire pressure would also play a part wouldn't it? We run 36psi front and 44psi rear per the manual, that higher pressure in the rear would act like a taller tire too. Put one tire 10psi and the other to 100psi and tell me they aren't different sizes. 36 to 44 psi is a 22% difference in air pressure from front to rear.

What about all the videos of guys doing burn outs, they cant all be RWD. If the mismatch is death, what's 4 minutes of donuts and drifting? That would surely destroy the diff, yet there are tonnes of videos on YouTube of just that.

This thread seems to be full of lots of information and user account, but no real data. Sorry I'm a data guy and the rule goes like this: "In god we trust, all others bring data." Am I missing the data?

 

no man... we're all just paranoid about our cars and anything that might somehow lessen their performance, or shorten their *lives*. though, having said that, it is unwise to run mismatched or old tires against new in these cars or in opposition to that 3% rule/myth/ internet rumour.. given there's always what? min? 5% power to the front? i don't think it's nitpicking to be keenly aware of what that front viscous coupler might or might not.. be doing.

PS... 36/ 40 or go on a diet. lol

 

Sounds like you are a big tobacco lawyer saying "there is no data that PROVED smoking caused YOUR cancer"

there are probably many ways to kill the diff. I wonder if it is possible to toast your diff and actually have no symptoms... are all the guys claiming "its fine dont worry' running around with non-functional couplings?

4 minutes of donuts, or 30 minutes of donuts is WAY diffrent than 10,000 miles or 200 hours of just driving.

Oh. on your argument re: tire wear and inflation pressures- do the ACTUAL math. it isnt diameter, but radius that causes the issue, and the reason the sizes need to be close to begin with is PRECISELY becuase wear and inflation pressures WILL cause variances past that nominal match. We dont know what the Porsche spec IS, in terms of how much mismatch they allow, but we know the nominal. So by matching the nominal you will still be within the design envelop.

And I seem to recall that Porsche calls for tread depths to be somewhat matched. I may be wrong on this, but from side to side I think they do....

How about asking that big porsche guy what the basis is for his claim that size doesnt matter? Does he have some data to support his assertion that there is a wide envelope and we are being silly internet rumor mongerers?

 

Haha, yeah i get what you're saying about smoking .

Regarding the diameter, starting with matched and wear taking them to the 3% difference isn't much different than starting with mismatched and having wear bring them to the same size really. Starting a 0% and going to 3% is the same as starting at 3% and wearing to 0%... ( so long as the wear brings them closer not farther apart ) With turbo cars the rears are always worn more than the fronts, if not you're not driving your car right

I hear you on the proof that it doesn't cause issues but shouldn't we have proof that it does first? That’s typically how things happen, it's not guilty until proven innocent usually.

Agree there are likely lots of blown front diffs, spend some time at the drag strip and you can see cars with viscous diffs do 1 wheel peals all the time. They do eventually wear out and thats it. Too bad we can't dyno on AWD dyno with the rollers unlocked to measure HP on F and R independently.Would be interesting to see if my diff is actually working and what power is going to it.

For disclosure, i do agree on matched tires, Porsche must have done it for a reason, but I'm not convinced we know why or if it does infact cause an issue if they are different. It's just a lot of hearsay that I can see.

 

abs/psm cels might well be, where hearsay ends, and real world experience with these issues begins. i've never had front diff issues, but i have certainly had mismatched tire issues that were corrected by more closely matching rolling diameters. eg new + new = good. old + new = cels, etc. they may be called idiot lights, but they tell us things we need to know. SOMEtimes

 

Hi folks,

I been reading through this whole thread and would like some advice before I replace my tires. I am currently running 235/35/19F and 315/25/19R on my iForged wheels. I believe the rears are 12" (not sure, because they came with the car).

My rear Pilot Sport PS2s are toast and the front still have 60% tread. These PS2s are just so expensive for the rear. Can someone here suggest some new sizes/brands I could run? Perhaps, the Hankook V12s? What would be the correct sizes? I would like to steer away from 315, since they are so expensive. Any recommendations would be greatly appreciated.

Thanks!

 

personally i would steer clear of the kooks ( at least the v12 ventus ) and stay with the ps2's.. they will help you steer better .. sounds like you only need rears so get some more 315's and when the fronts are toast, change them all as needed. thats what i would do.. if you have enough tread on fronts still

 

Yep, that's what I am doing. But since these suckers are so expensive and I am getting an alignment and corner balancing after this, I was thinking of changing all 4 right now, instead of waiting for the fronts to be toast. I can get 4 tires for the price of 2 PS2s. Any suggestion on brands/sizes?

Yes, the V12s def. aren't as good. Had them on my e46 M3.

 

i'm in a similar boat. i'm leaning toward yoko advans.. or possibly even the kook rs3' but i have size availability issues since i'm running a gt2 setup ( fatter 18's ). i may have to bite the $$ bullet to stay with the michi's all around again? but they are the better tire i *think* so worse things can happen. i have not tried the others i mentioned..*yet*, but don't do the v12's unless you only tool around the city.. in that case, they'll be fine. my .02

 

Yea, I just drive around the city and occasional canyon run. What sizes on the v12s would you recommend, without mucking up the diff?

 

For what its worth, I had the same problem on ours, could not find 315's. We switched from 315 to 305 PS2 which are readily available. Keep in mind the 305 is 1 size taller tire. I specifically asked the head Turbo technician at Newport Porsche and he said it is all hog wash, the coupler will never know the difference. I also sent an email to Porsche Germany asking the same question, they came back and said that slight differences in diameter, like 1 tire circumference different, will have no long term affects on the coupler and will not affect performance.