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Discussion Starter · #1 · (Edited)
Hello guys,

I just purchased a 2006 Cayman S last month and so far I am loving it! I currently have 19" Carrera wheels (stock tires - 235/35/19F & 265/35/19R) and OZ Ultraleggera HLT (18x8.5+50F & 18x10+37R this is with 5mm wheel spacer front and back) wheels. I am planning to use my OZ Ultraleggera as my autocross wheels as soon as my Bridgestone RE71Rs are worn on my stock 19s. When that happens, I would like to stuff as much rubber I can possibly do. Question - Can I run 265/35/18 and 285/30/18 without having any fitment issue and no error message? The reason why I am planning to use the size mentioned is that Bridgestone RE71Rs doesn't come with a lot of size option for 18s. I have heard that the ABS and PSM can act weird when putting a different size tires on the car. Do you guys know what causes these issues? Is it the ratio between the front and rear tires' diameter or is it the ratio between the tire that you want to run to the OEM size?

Thanks in advance!

PS: I tried researching about this but I can't find a clear answer. Thanks again!
 

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I'm running 255/35-18 front & 275/35-18 rear RE-71R on the same size and very similar total offset Forgestar wheels on my 2010 987.2S with great results. I've only had a few events on these new wheels, but I've been running these same size tires (ZII, ZII* & RE-71R) on OEM width 8x18/9x18 wheels for about three years.

You need to be very careful about rake (f:r) height ratio when changing sizes because watching the difference in rotational speed is now ABS/PSM detects wheels slipping and circumference ratios that differ much from OEM can cause ice mode engagement or other shenanigans. The size I run (255 & 275) matches OEM rake within 1/4" height, and is within the OEM tolerance for tread wear. 265/35-18 & 285/30-18 will be about 0.75" off from OEM rake and may cause problems.
 

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Discussion Starter · #3 ·
Thanks for the reply, sjfehr!

What is the recommended ratio difference between the front and rear? And what is the recommended percentage difference between the circumference of the tire that you want to run compared to the OEM? Also, what do you mean by rake?
 

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Rake is the ratio between front and rear tires. OEM rear tires are 3.7% taller than the front, and the computer expect to always see the fronts turning 3.7% faster within a tolerance for tread wear. What I usually do is compare the tires I'm considering to the OEM tire sizes and make sure they both going up or down about the same amount. Conventional wisdom is to stay within 2-3% of OEM rake, but I don't think there's been any serious testing about it besides anecdotal complaints from people with tires 3%+.

OEM rear tires (265/35-19 & 265/40-18) are 334mm tall (give or take a half mm). 285/30-18 is 314mm tall, about 7/8" shorter than OEM. You'd get better gearing for acceleration, which would be a pro. You'd need to match that with something similar on the front like a 265/30-18, which unfortunately just isn't available. Which is why I settled on 275/35-18 (3/8" shorter than OEM) rear and 255/35-18 (3/16" shorter than OEM) front. This keeps rake within 1% of OEM and I've never had any issues.
 

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Discussion Starter · #5 ·
Sweet! Thanks for explaining!

Now, it makes more sense! My other option is running RS4 265/40/18 and 295/40/18 but I am afraid that I might not have the clearance in the rear even with the 5mm spacer that I have. Thoughts? Also, have you heard anything about the RS4s?
 

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You shouldn't have PSM issues with that combo, but they'd be about 1/2" taller than OEM and may not physically fit without rubbing, spacers or not.

R-S4 tests thus far have found them to be slower than RE-71R but those were cold weather tests where they never had a chance to come up to full temp. If they're anything like R-S3, they'll need heat to work.
 

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Discussion Starter · #7 ·
Yea, I am worried about the fitment. I wish Bridgestone will make more sizes. Oh well, I am just going stick with 255/35/18 and 275/35/18. Thanks again!!!
 

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Discussion Starter · #8 ·
sjfehr,

Someone replied to my thread on the other forum and he said this

"I am currently running exactly as you describe...RE71R in 265/35 front and 285/30 rear. With SCCA legal offsets and 8.5/10" wheels there is no rub at either end. The car has been driven at interstate speeds on the street with no complaining from PSM at all and I've noticed nothing with braking either.

For what it's worth, the 285s come tantalizingly close to mounting on some 8.5" front wheels. We could not quite get them to seat a bead...but it seemed so close that we tried for hours. If the tires weren't brand new or you could leave them out in the sun for few hours...they may actually go on."

He has a 2006 Cayman S
 
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