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      02-20-2017, 09:15 AM   #1
mdai245
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Best tire sizes for ec7 18x8.5 square on AWD

Im thinking about doing some HPDE with my new apex ec7 wheels using a square setup. I have narrowed down my tire size selection to:

235/45 18
245/40 18
255/40 18

255/40 is the closest to OEM rolling diameter and has more grip but weighs more and the extra grip might not be needed with awd. Its also max wide for 8.5 width wheel which may cause sidewall flex in corners.

245/40 seems very common size for track tires and a near perfect fit for the wheel but its heavier than 235 and odometer will incur 1% more milage due to smaller rolling diameter.

235/45 is the lightest, will offer the most sidewall protecrion from potholes, and will read 1% less milage due to larger rolling diameter.

I would want to go with 235/45 but tire selection is limited and im not sure if 235 is wide enough for track duty or it would provide good handling characteristics with its taller side wall.

What do you guys think?
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      02-20-2017, 09:55 AM   #2
Polo08816
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Quote:
Originally Posted by mdai245 View Post
Im thinking about doing some HPDE with my new apex ec7 wheels using a square setup. I have narrowed down my tire size selection to:

235/45 18
245/40 18
255/40 18

255/40 is the closest to OEM rolling diameter and has more grip but weighs more and the extra grip might not be needed with awd. Its also max wide for 8.5 width wheel which may cause sidewall flex in corners.

245/40 seems very common size for track tires and a near perfect fit for the wheel but its heavier than 235 and odometer will incur 1% more milage due to smaller rolling diameter.

235/45 is the lightest, will offer the most sidewall protecrion from potholes, and will read 1% less milage due to larger rolling diameter.

I would want to go with 235/45 but tire selection is limited and im not sure if 235 is wide enough for track duty or it would provide good handling characteristics with its taller side wall.

What do you guys think?
I have the ARC-8 in 18x8.5 and I use 255/40/18.

It maintains the stock sidewall height so there's slightly more protection against potholes. But wider doesn't = more grip. It's a common misconception.

Grip = friction

Friction = coefficient of friction * force

Changing your coefficient of friction will have the largest effect on friction (grip). You can alter force by taking advantage of weight transfer.
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      02-20-2017, 02:21 PM   #3
mdai245
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Yeah there are a lot of variables at play when it comes to lap times and road holding. Better compound can of course increase friction and give more grip.

I want to know if 235/45 wide tires will have enough road holding capability at the limit vs 255/40 with the same tires on a AWD. The AWD doesnt slip nearly as much RWD when launching the car, but I am concerned about whether those 20 mm will save me from a spin out or hitting a barrier when taking corners at speed, otherwise I can save 2 lbs per corner and enjoy better milage with the 235/45. This is my DD and I dont want to race the car, just enjoy it at a few HPDEs.
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      02-20-2017, 04:01 PM   #4
Polo08816
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Quote:
Originally Posted by mdai245 View Post
Yeah there are a lot of variables at play when it comes to lap times and road holding. Better compound can of course increase friction and give more grip.

I want to know if 235/45 wide tires will have enough road holding capability at the limit vs 255/40 with the same tires on a AWD. The AWD doesnt slip nearly as much RWD when launching the car, but I am concerned about whether those 20 mm will save me from a spin out or hitting a barrier when taking corners at speed, otherwise I can save 2 lbs per corner and enjoy better milage with the 235/45. This is my DD and I dont want to race the car, just enjoy it at a few HPDEs.
Those 20mm are not going to be the difference between you spinning out or not.

You won't necessarily get better mileage with taller tires either.
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      03-20-2017, 08:33 PM   #5
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Quote:
Originally Posted by Polo08816 View Post
I have the ARC-8 in 18x8.5 and I use 255/40/18.
did you need to do anything to accommodate the 255 width tires in front?
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      03-21-2017, 04:57 AM   #6
Polo08816
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did you need to do anything to accommodate the 255 width tires in front?
Negative. No spacers or camber adjustments needed.
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