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Discussion Starter · #1 ·
Hello,

I'm changing the rear wheel size from 18x9 to 18x10. I plan on sticking with the stock tire size (265/40) and am fine with a more "stretched" look, but wonder, will the final diameter change based on the rim width?

If so, would a 275/40 on a 10 inch rim match the diameter of a 265/40 width on a 9 inch rim?

Thoughts?
 

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No, tire stretching won't change the final diameter of the tires, that can affect speedometer reading/gear ratios. It will, however, change the ride quality: more stretching equates to firmer ride.
Changing from 275/40 from a stock 265/40 will have an affect on speedometer reading/gearing ratios.

Also, increasing your contact patch dimension by using wider tires, doesn't always equal to getting more grip. Weight of car and coefficiency of friction due to weather/rubber compound/etc determines the grip values. You will get more grip on a 275/35 than you will on a 275/40 due to higher pressure loads on the lower profile tires.

For more precise calculations on the effects to the speedometer and gear ratios, as well as effects on tire stretching/compressing, you can read more about it here:
Car Bibles : The Wheel and Tyre Bible Page 3 of 4
 

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How would mounting a 265/40/18 on a 10" rim alter the tire diameter?

The 275/40/18 is about .3" greater in diameter than the 265/40/18 (26.6" versus 26.3").
 

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Also you might want to take into consideration that there are no N-rated tires for the 275/40R18 sizes. Meaning that they're most likely considerably heavier.

N-rated tires (N0, N1, N2, N3, etc) of the same model are different because they're specifically revised for Porsches. The higher the N number the more recent is the revision for the same model tire. For example, on a good year F1 Assimetrico a 235/40R19 (N0) is 20 pounds, while regular F1 Assimetrico non-N rated tires 235/40R19 is 25 pounds. 5 pounds difference in weight that on rotational dynamics translates to 30 to 40 pounds stationary weight equivalent.

For your Forgestars F14, if you're on a base Cayman, I would stick to the 265/40, and make sure that you're on an N-rated tire set up, and choose the lightest option in weight (Continentals or Michelins) as heavier tires will be counter productive to the F14s light weight advantage and rob the car from its acceleration.
 

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Discussion Starter · #5 ·
Thanks for the responses. I was thinking the diameter would change by assuming that a wider rim would result in a lower sidewall height because the tire would be "stretched". If the sidewall was shorter due to a wider rim, or taller (bunched up) on a narrower rim, I thought it could change the overall effective diameter of the rolling tire...

A narrower rim with the same tire = greater diameter
A wider rim with the same tire = smaller diameter

But it looks like the diameter doesn't change, which is good to know!

Regarding tires, I was thinking of the V12's, but was wondering how 95 load capacity for the fronts would be different from the 91 load capacity of stock. I think I already save 4-5 pounds per corner just by changing wheels!
 

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Typically, a higher load index will mean more reinforcements on the sidewall to allow the higher weight range. (91 @ 1356lb and 95 @ 1521lb) and therefore if a higher index tire is used on a car that normally only requires 91 index, it will be firmer, and synonimous to better handling at the cost of comfort.

However since rubber compounds varies, and that too has an effect on comfort, I would recommend asking people who's used the V12s personally. And from what I've read, they've only got good things to say about it, especially regarding noise levels.
 

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Hey vrecaro,

Thanks for the explanations! That was very helpful.

-Moto
 

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If you are planning to transplant your OEM tires to your new rims, I can tell you that there isn't that much noticeable stretch, if any at all.

I moved from 18x8 and 18x9 OEM wheels to 18x8.5 and 18x10 aftermarket wheels and kept my OEM Continental tires. Neither my front nor rear tires look stretched at all. In fact, I can even lay the wheel + tire face down on the ground and the tire will be the only thing touching the asphalt.
 
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