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An acquaintance of mine is selling the winter tires off of his 911. The sizes are 245-35-20 and 285-35-20. That's a diameter difference from OEM 718 Cayman (235-35-20 and 265-35-20) of +7mm front and +14mm rear.

I've asked around and our Porsche techs seem to think it will not be an issue. The tires will certainly fit and not interfere with steering or suspension. At the same time, our Porsche Service Manager seems to think it's not worth the potential for a headache down the road. He thinks the PSM will not work at its ideal level due to the imbalance of size from OEM.

Me personally, I like the chunky tire look for winter. I really want to use the 20's year round which is why I bought them in the first place. And, a +7mm difference from the rear compared to the front doesn't seem like much. That's like worn rear tires, right? Heck, we have guys with race tires that substantially vary from the OEM ratio.

Yes, I can get a great deal on a 19" winter wheel/tire combo, but if this 20" tire combo makes sense, this is my preferred solution.

Thoughts?
 

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Did you ever try this combo out? I"m thinking of trying out 245 or 255 front and 275 or 285 rear on stock rims. I like meatier tires and sidewall too.
 

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The newest issue of Car and Driver has the 2020 Cayman GT4 on the cover. Inside, they say the car comes stock with 245/35-20's on the front and 295/30-20's on the back. Also, the article says the GT4 sits one full inch lower than the stock Cayman. Anyone see why we couldn't put 245/35's and 295/30's on a CGTS with no worries about rub/interference?
 

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The wheels on the GT4 are a different width and offset from what's on the other variants, so you may not like the result of stuffing the GT4-sized tires on your GTS wheels. The fronts aren't too different, but the rears are the challenge (265 vs 295).
 

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Just be careful going wider on winter tires. Regardless of the fit issue, I’ve always understood you want to go NARROWER on winter tires. Wider is like skis and will sit on top of the snow and provide less traction. Narrower tires will have more weight per contact patch and thus cut through and grip better. Don’t skimp on a “proper” winter setup. One accident or one incident stuck going uphill with a line of cars behind you and you will regret it. Safety first in this case! Personally I -2 downsized to 18” all around going I think 20mm narrower and the car was fantastic through the snow. I know this is not what you asked for, but I just wanted to chime in mostly based on the safety concern. Every situation I know is different however.
 
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