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2008 Cayman S and 2010 GT3
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Discussion Starter #1
On the fence... 275-35-18 or 40... 40 can look a bit chunky and tall with some brands... fronts are 235-40’s
 

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It should fit but I remember the guy at a track recommended me to use 245/40/18's for the front to avoid understeer.

I just did the tire mounting of my 275/35/19's and they fit fine.
 

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2008 Cayman S and 2010 GT3
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Discussion Starter #3
I’m not putting a 245 on an 8” rim, but thx for the 275-35 info.. fitment wasn’t the question really, more so does it look to small vs 40 side wall On (18’s)
 

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I use 245/35/18 f and 275/35/18 Michelin PS4Ss on my OEM wheels which would work for you just fine. I also have two other wheel/tire setups, Titan 7 TS-5 which are light weight forged monoblock racing wheels, 9 in in f and 10 in wide in the rear with RE71R's 255/35/18f and 275/35/18 f. The third set is a Signature SV-104 forged monoblock racing wheel with Hoosier A7's 255/35/18 f and 275/35/18 r. My thought was to get all three of these wheelset and tires to match my cars setup, so when changing wheels/tires, I would not need to change the swaybar balance. The tire footprint of all three do that nicely. However, now I know that I could have chosen one width size larger for the race setups, My next setup will be 265/35/18 f and 285/35/18 r. This sizing should give me more stick/traction while having the same roll rate balance f to r.

So, currently both my two race sets 4 wheel drift evenly. Have good turnin and good throttle traction response from just before the apex through the corner exit. I do have a Wavetrac LSD to help manage this tire patch traction/roll balance setup.
 

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Discussion Starter #5
Thx for that! So would 235-40 and 275-35-18 work? Or should I get 275-40’s? In other words is mixing side %’s a no-no?
 

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The side walls are actually part of your suspension. They have spring rates, so you need to keep those things in mind when changing the sidewall ratio. The common upgrade is a wider version of the OEM setup with a shorter sidewall ratio, (meaning higher spring rate/better turn in and mid corner traction control). I really like the 35 ratio tires, especially the RE71R then a couple of steps down are the Michelin PS4S. So maybe a 245/35/18 f and a 275/35/18 r Michelin PS4S would work for you on your OEM wheels. I really like that combo for my daily drives, comfort and noise are much improved over the RE71R's. However, the traction, turnin, mid corner power transfer to the road is much better with the RE71R's. A lot of people who want performance from their cars, use these tires daily. They are noisy and harsh (higher sidewall spring rate) though. Pictures are with 18 mm spacers f and 23 mm spacers r.

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