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Cayenne wheel/tire setup for snow

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Hi, I would like to know if anyone has any thoughts on this. Just bought a new Cayenne v6 and it came with 18' wheel upgrade. I have a brand new set of 17' Blizzaks I bought for my wife's X5 last year with only around 1000 miles on them. I am assuming I may get better snow performance with a 17' rather than an 18'. The best bet may be to find a set of Cayenne 17 wheels and use them every winter. I would like to know if anyone has anyone experience with this. Keep in mind we get a LOT of snow.

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I found a set of 17's on ebay that were 'take-offs' from a Cayenne that was upgraded to a bigger wheel size. I have my factory set of 19's for summer and the 17's for the winter use.

If I'm not mistaken you may also have spacers on the 18's that need to be removed as well when mounting the 17's.

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Improved winter performance is more a function of the *width* of the wheel/tire, not the *diameter* of the wheel. This is because a tire that is narrow results in a contact patch with the ground that has a smaller surface area.

If we make these assumptions:

1) Weight of vehicle is constant (4400 lbs)

2) Front-to-back 'length' of contact patch is constant (8 inches)

3) All 4 wheels are the same contact patch, and receive the car's load equally)

Thus, a 7-inch wide tire will result in 4 contact patches that are each 56 square inches, and each bear 1100 lbs of load. The resulting pressure of that contact patch is 19.64 psi.

If you had an 8-inch wide tire, the surface area increases to 64 square inches, with a resulting pressure of 17.19 psi.

Therefore, changing from an 8-inch wide tire down to a 7-inch wide tire will increase the pressure on your contact patches by 14.3%. This is equivalent to adding 629 lbs of weight to the vehicle. That's a *lot* of kitty litter!

Since the force of friction per square inch (how much grip the tires will have) equals the contact patch pressure times the coefficient of friction, skinny tires will have much better grip on wet, icy roads. It's amazing how much difference an inch of width can make.

But your logic is right on. As a general rule, 17-inch rims are narrower than larger diameter rims for the same vehicle. And of course, 17-inch rims and snow tires are also cheaper than 18s or 19s. Plus, they tend to get more beat up in the winter, so why not beat up on the cheap ones.

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Thanks for all of your input. I have been able to find some Cayenne factory 17" wheels. My next question is -Will a set of Blizzak 235/65 17s fit these factory 17'? I am assuming they will however the calipers on the 18's look big and wondering what if any problems will I run into? I have a set of nearly new blizzaks leftover from my wife's X5 before we traded for the new V6 cayenne. I think the V6 cayenne will ve a nice suv for my wife this winter, we get a lot of snow here in upstate NY and looking forward to the added safety/security of the low range. Thanks in advance for your help.

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