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Analysis Paralysis on C4S tire sizing

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Old 03-15-2017, 05:31 PM
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Ken Allwine
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Default Analysis Paralysis on C4S tire sizing

I could use some help here.

I am having 19x8.5" and 19x11" Forgestar wheels built for my 04 C4S. So, obviously, I'm moving to 19" tires.

From the reading I have done, I have to take into consideration of the tire speed in relation to front and back because of the viscous coupling for the AWD system.

Factory tire size:
F: 225/40-18
R: 295/30-18
Roughly looking at revolutions per mile charts and tire diameter charts, the front tires rotate approximately 4.007% faster than the rears

19" tires
F: 235/35-19 (for 8.5" width wheel)
R: 305/30-19
This would equate to the front tires rotating more per mile by a 2.95% difference.

Would this drop in difference cause a problem with the drivetrain?
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Old 03-15-2017, 06:30 PM
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kmagnuss
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Look over in the 996tt section... there's a 10 page thread on this.

FWIW a lot of guys including myself run 235/35/19 with 315/25/19.
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Old 03-15-2017, 07:00 PM
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Originally Posted by Ken Allwine View Post

19" tires
F: 235/35-19 (for 8.5" width wheel)
R: 305/30-19
This would equate to the front tires rotating more per mile by a 2.95% difference.

Would this drop in difference cause a problem with the drivetrain?
You need the rear tires to rotate faster than the front. The rear tire has to have a smaller diameter. The set up you have will seldom direct any power to the front wheels.
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Old 03-15-2017, 07:39 PM
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rear should be 305/25 for 11s. if you had bigger 11.5-12, 315 would be ok
front should be 255/35 for 8.5
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Old 03-16-2017, 11:55 AM
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Ken Allwine
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Would a 255 even fit on an 8.5" wheel? Seems pretty wide.
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Old 03-16-2017, 12:30 PM
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Originally Posted by Ken Allwine View Post
Would a 255 even fit on an 8.5" wheel? Seems pretty wide.
yes, easily
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Old 03-16-2017, 12:45 PM
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This shroud have been determined BEFORE commissioning the rims. My suggestion is to stay as close to the OE ratio as you can.

As a side note, all the racing watercooled 911s that I've seen had 18" rims. None went bigger than that. Something to think about when looking for performance enhancements in the future.
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Old 03-16-2017, 12:50 PM
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255 would be the max I think. It could also affect driving characteristics.
Much depends on which particular tire you will choose as each of them has a different shoulder profile.

For example ... I had a 295/35/18 Bridgestone S02A on a 10J rim which was way to big. The tire lost its squareness at the shoulder. Which influenced cornering stability. However when I put on the Yokohama AD08r295/35/18 it looked much more tight and square then the Bridgestones. Most probably due to its extra stiff and stronger carcass.

So each tire type will cope differently with the extra width. Me personally, I would not go wider then 245. Moreover the balance between width in the front and rear is also important. So it depends also on that. But of course each person has its preferences and if it is for cruising around then it is less important.
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Old 03-16-2017, 12:58 PM
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245 would be the correct width for a 8.5 wheel
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Old 03-16-2017, 01:05 PM
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Not helpful for your question, but I would love to see pics of your 4s once you have the forgestars on as I too have been contemplating trying these.
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Old 03-16-2017, 06:39 PM
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I found a set of OEM 4S wheels for cheap - had a bunch of scuffs; but wanted to run these for track days.... keep my 4S wheels in the great shape they're in. This way I didn't have to speculate on ratios and sizes, etc

I will be running the Toyo Proxes R888R this weekend.
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Old 03-17-2017, 10:01 AM
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Ken Allwine
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Originally Posted by Imo000 View Post
This shroud have been determined BEFORE commissioning the rims. My suggestion is to stay as close to the OE ratio as you can.

As a side note, all the racing watercooled 911s that I've seen had 18" rims. None went bigger than that. Something to think about when looking for performance enhancements in the future.
I am staying as close to the ratio as possible. Also, my car will never see the track. I'm not really into that side of things. My C4S is a nice weather driver and "cars and coffee" regular. It probably sees less than 3000 miles a year. So I don't consider the wheels a "performance enhancement".

This only came to the surface when I was looking through the forums and ran across the tire dimension conversation and the differential. I was not aware of the rotational speed difference between the front and back. I assumed the overall tire circumference was the same front to back.
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Old 03-17-2017, 10:02 AM
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Ken Allwine
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Originally Posted by HNSC4S View Post
Not helpful for your question, but I would love to see pics of your 4s once you have the forgestars on as I too have been contemplating trying these.
This is the wheel I chose. Forgestar F14

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Old 03-17-2017, 11:11 AM
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Originally Posted by Ken Allwine View Post
This is the wheel I chose. Forgestar F14

looks great
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Old 03-17-2017, 11:50 AM
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Originally Posted by Ken Allwine View Post
I am staying as close to the ratio as possible. Also, my car will never see the track. I'm not really into that side of things. My C4S is a nice weather driver and "cars and coffee" regular. It probably sees less than 3000 miles a year. So I don't consider the wheels a "performance enhancement".

This only came to the surface when I was looking through the forums and ran across the tire dimension conversation and the differential. I was not aware of the rotational speed difference between the front and back. I assumed the overall tire circumference was the same front to back.
There is an allowable variance in all the dimensions between tire manufacturers. Go on a site that states actual tire diameter for a specific tire that you are looking to buy and then calculate the ratio. This way you'll know exactly what you are buying.
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