Can I run 295/35 (instead of 30)/18 on my C4S?

Last month in Excellence Magazine I saw an ad for some Nitto Invo tires that caught my eye. It's time to replace mine so I thought I might give them a shot. Unfortunately they don't make the OEM 295/30-18 rear tire size. Kinda odd since they are advertising in a Porsche magazine. I called their customer service to see if they were in development, but they weren't helpful. So... Can I substitute a 35 instead of a 30? Or I should say, has anyone done this and how does it affect handling? I know the sidewall will be a little taller than spec, but not sure what impact it will have on how the car handles. Thanks in advance for your feedback!

How about 315/30 instead for the rears and 235/40 for the fronts? That will "fit" better than thicker sidewall for the rears

I would do that, but the widest they make in 18" is 295. I guess I need to step up to a nice set of 19's like yours! Great looking car!

See if you can get measurement of the Nitto's true tire rollout measurement & compare to that of the tire you're using now (of course you'll need to know the rollout measurement when new to be consistent) - I know when I had ContiSports a few years ago in a 285/35 configuration, then went to Yokohama's in 285/40, the rollout was less than 1/3" difference. I had those tires on an older MB CLK and noticed no difference - then again, with that car "performance" didn't really apply when the road got twisty. I may get flamed for this but, see if they have a 275/35

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Thanks! That's unfortunate they don't make the 295/30/18 for the model you want. I do know that Yokohama makes the 295/30/18 for the Neova/AD07 model... which a very nice tire.

You'll be fine with it. It has a RD of 26" versus the 25" of the PS2 that comes on the car in 295/30. Your gearing will end up being a little taller but no biggie unless you are on track and then it is a big deal. Also, your speedo could read a few mph slower than you are actually driving which with the variance we already know these cars have it could be spot on What size front are yo going to run? The 235/40 I assume?

Thanks Dell. I'll probably end up with the PS2s since I do track the car ocassionally. I've got P Zeros on there now and have actually been pretty happy with them, but want to try something a little different. I've heard great things about the PS2s on the track and street.

PS2's are a GREAT tire for the track. I used them for some time on my GT3 before moving to r-comps. Great grip and very communicative.

To keep ABS and mainly PSM happy, it's suggested to keep the hight difference between front and rear within 3% so depending on your front tires, the 295/35/18 will work from this aspect. However, if the car is lowered and if your rear wheels have an aggressive offset, you might get some rubbing. I currently run 285/35/18 rear tires (Pirelli P Zero Rosso) on my 996 NB (front 225/40/18 - just below 3% hight difference - no issues with ABS; I've no PSM) and run an offset of 53 in the rear on a 10 inch wheel. My car is lowered to GT3 stock hight and I have occasional rubbing. I'd definately get rubbing with a 295/35/18 tire. I just ordered new tires and didn't want to go to R compounds as of yet but as low as possible for the treadwear rating so I came accross the Falken Azenis RT-615. They don't offer a 285 tire and only a 295/40/18 so I'll get a 275/35/18 for the rear and a 235/40/18 up front. These tires are less than 1% different in hight. Their treadwear rating is 200 (R compounds have 100). http://www.falkentire.com/xhtml/tire...egory=2&tire=2

Yes, I agree with the 3% deviation. It will eventually keep ABS/PSM happy and your differential happy too.

Sure, 3% is great but with the tire he is considering we are talking maybe 4.5% tops. Not a big deal in the grand scheme of things.

Looks to me like your car is a C4S. ABS/PSM are the least I'd worry about. I would make sure you're 4% or under to avoid potential damage to the front diff given it's 4K plus to replace it.

Maybe some of y'all quoting 3% and 4% figures can produce the PAG/PCNA literature that states where they require the variance threshold to be at????

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Ah, the slippery slope has started. Next is another set of rims. Then R-comps. Then PSS9/10/JRZ/Moton. Then ditch that car and get a GT3. Then upgrade GT3 to Motons. Then ditch the GT3 and get a race car. Might as well skip the 150K in the middle and go straight to a race car now Oh wait, were we talking about tires? -td

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Yes it has... Already moved to PSS9s and love them. Luckily I have a wife that is supportive but provides reality checks from time to time. Therefore, no race car...for now. I appreciate all the advice. I'll probably stay with OEM specs and do PS2s unless someone can convince me that I'd be better off running a wider tire on my stock rims.

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Here's a quote from Continental that recommeds to stay within 3% of the original equipment rolling circumference: http://www.conti-online.com/generato...lus_sizing.pdf "Rolling Circumference: The Original Equipment rolling circumference should be maintained as closely as possible. The rolling circumference of the original equipment tires is a critical aspect of the tire fitment. If the original equipment tires are replaced with tires that have a rolling circumference different from the OE fitment, it could affect items such as the speedometer calibration, anti-lock brake systems (ABS), stability control systems (ESP), etc. Under most circumstances a change within plus or minus 3% does not require any adjustments to the above mentioned systems. If there is any question about the suitability of the eplacement tire size, please contact the vehicle manufacturer and follow those recommendations. For All Wheel Drive and 4 Wheel Drive vehicles, Continental Tire recommends replacing all tires on the vehicle with the same size tire unless the vehicle came from the vehicle manufacturer with a �staggered� tire fitment." The here suggested tires are: 295/35/18 (diameter 26.12 inch) 295/30/18 (stock, diameter 24.96 inch) Variance 4.6% so more than 3%. The stock set-up variance between front and rear is less than 1% so sticking to stock front tires and increasing the diameter of the rear tire by 4.6% leads further to a variance between the front and rear tires of more than 3%. There might be some input from Porsche but manufacturers usually don't suggest to use other than stock tire sizes so there's really no need for Porsche to comment on this. It's more the tire industry that wants to sell other tires so they have to mention this requirement. It might further be found in the communication of the system manufacturers like Bosch (ABS, PSM).

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While I understand the variance percentage and the POSSIBLE implications involved with a 4WD car, I most certainly disagree with this statement that there is no input needed from Porsche. Their ABS controllers are specific to their cars and models. You would need to consult a manual for TSB, etc, to get clarification from PAG/PCNA.

Here is a pretty good tire sizing calculator http://www.1010tires.com/TireSizeCal...?action=submit

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Seriously, I highly recommend getting another set of rims and putting your track tires on those. Then you can run Sumitomo or Yokohama cheapies (with high treadwear) for the street. They're a good tire, and a nice bargain - especially if you have negative camber dialed in. And you won't fudge up your good rims on the track. Also, if you get take-off track tires from John Berget or someone else, you can save a bundle. I recently got my last SET of MPSCups (locally) for $200 - with 3/4 tread on them. My last set of RA-1s I got for $25 apiece. And those lasted 20 track days before the rears corded (could have gotten more if I flipped the rears, but I missed that window of opportunity). I got both sets here in Dallas (MSR). I think the R888 (and maybe Nitto 01s) should be coming to the "after" market pretty soon - if they haven't already. -td

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Thanks. Those sound like fantastic deals. I may start keeping an eye out for some used wheels for the track. Any idea who I should speak with at MSR to put my name on the list for take-off tires? Isn't John Berget in WI. Is shipping cost prohibitive?

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I actually found my last set of MPSCups here (Rennlist in the Racing Forum). I don't think there is a take-off list at MSR, although PST might be able to keep tabs on tires. I haven't used John yet, but all the folks that I've talked to that did have had positive things to say. -td

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Maybe I've bought into some urban legend, but I swear I've read a "legitimate" document on the subject at some point. Thinking about it, the reality is Porsche will not comment due to the fact they already have in the form of "only X tires and sizes" are approved. The only stipulation from Porsche I've found that comes close relates to replacing a tire. Essentially, the replacement tire and opposite tire on the same axle must be within 30% tread depth of each other. I for one will continue to follow the logic that they should be as close as possible in rolling diameter.

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It's not just Porsche, BTW. MB has a similar 3% guideline to their AWD vehicles, as do BMW.

I would not use 295/35/18.. It's too big with 235/40/18 Stay with 295/30 or 315/30.

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"too big" doesn't define anything. You really need to get the tire manufacturers specs on the RD to see what it REALLY is and then compare to what the factory RD is. Tires with the SAME size can still have very difference RD. Of course nobody is saying to run a tire that is wildy larger or smaller than stock but you have some wiggle room. If you want to follow the letter of the law to a T then contact PCNA and get their OFFICIAL stance on variance between F&R and R&R.

Nitto Invo: 295/35/18 OD is 26.02 235/40/18 OD is 25.39 That means they have a OD diff of 2.5% with the rear tire being larger. It's beneficial to actually have the rear slightly smaller to rotate a little faster which allows a slightly higher bias on the diff to turn the front wheels. Now you can run 245/40/18 (25.75) for the front which will match better than the 235, but it'll start to rub on the inner front fenders under full lock.