Need exact Tire Diameter
Thread Starter
Heel & Toe
Joined: Nov 2007
Posts: 16
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Need exact Tire Diameter
I need the exact tire diameter for the tires on a 2008 C6 6-Speed Manual with a Z51 package. I am calculating Max MPH in each gear to decide what rear axle ratio to put in. Any help is appreciated.
Drifting
Joined: Nov 2005
Posts: 1,745
Likes: 35
From: Turlock CA
To answer your question -- it's 26.85" for the 285/35/19 tires (285/25.4 * 0.35 * 2 + 19).
--Dan
Le Mans Master
Joined: Feb 2006
Posts: 5,762
Likes: 4
From: Chandler Arizona
I've always calculated it exactly the way Dan did. But it does give you a small error since tires squat a little. The most accurate way to calculate the exact speed is to go to tirerack.com and get the revs per mile for your exact rear tire.
Tirerack lists the Supercar 285/35-19 as 26.9" in diameter and 775 revs per mile. If you use the 26.9" diameter you would get 750 revs per mile, so you can see you'd actually be 3% off on your calculations.
Tirerack lists the Supercar 285/35-19 as 26.9" in diameter and 775 revs per mile. If you use the 26.9" diameter you would get 750 revs per mile, so you can see you'd actually be 3% off on your calculations.
Drifting
Joined: Nov 2005
Posts: 1,745
Likes: 35
From: Turlock CA
I've always calculated it exactly the way Dan did. But it does give you a small error since tires squat a little. The most accurate way to calculate the exact speed is to go to tirerack.com and get the revs per mile for your exact rear tire.
Tirerack lists the Supercar 285/35-19 as 26.9" in diameter and 775 revs per mile. If you use the 26.9" diameter you would get 750 revs per mile, so you can see you'd actually be 3% off on your calculations.
Tirerack lists the Supercar 285/35-19 as 26.9" in diameter and 775 revs per mile. If you use the 26.9" diameter you would get 750 revs per mile, so you can see you'd actually be 3% off on your calculations.
--Dan
Le Mans Master
Joined: Feb 2006
Posts: 5,762
Likes: 4
From: Chandler Arizona
That is really weird. You wouldn't think that the tires would be that different being that they are both stiff runflats. This means that either the standard runflat squats more or grows less at speed or a combo of both effects.
Le Mans Master<br><img src="/forums/images/ranks/5k-6k.gif" border="0">
Joined: May 2007
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St. Jude Donor '08-'09
I measured both, unweighted and they both measured 26.7". I did this after using the tirerack.com statistic and not getting my speed adjusted right. Once I measured it made sense.
Tech Contributor
Joined: Mar 2003
Posts: 12,094
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From: Colorado Springs, CO
St. Jude Donor '08-'09-'10-'11
As for track....128 is the top with 4.10's. Thats high 10's. If you plan on mid to low tens get 3.90's. It will require forced in duction since no car that is head/cam ever got under 10.8. N2O can do it too. Dont get a gear set because you think its better for traction to get 3.90's instead of 4.10's.
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Drifting
Joined: Nov 2005
Posts: 1,745
Likes: 35
From: Turlock CA
GS2s w/805 rev/mile give a 25.05" diameter vs. the listed 26.9".
Supercars w/775 rev/mile give a 26.02" diameter vs. the listed 26.9".
Something is off.
Having less tread (on the Supercars) would result in a smaller diameter (in theory) and higher revs/mile.
--Dan
Team Owner
Joined: Feb 2001
Posts: 27,253
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From: Short Hills, NJ
I've always used the 'width x the sidewall % x 2 then divided by 25.4 + the wheel diamter' method as well and had come up with 26.85 for our rear tires. But the above info is also critical in determining the true overall diameter.
Also consider tire wear when making these calculations for a tire with 15,000 miles on it will not be quite as tall overall as one with 15 miles on it.
Also consider tire wear when making these calculations for a tire with 15,000 miles on it will not be quite as tall overall as one with 15 miles on it.
Drifting
Joined: Nov 2005
Posts: 1,745
Likes: 35
From: Turlock CA
HP Tuners Speedometer Settings
Got to thinking and looked at my tune in the Speedo area. OEM (A6 non/Z51) mine was set to a tire size of 26.80" and the 2.56 gear.
Anyone with a manual and/or Z51 want to post what your settings are? We can see if GM is allowing for the different posted tire specs. And if not, perhaps the listed specs at Tire Rack are just wrong.
--Dan
Anyone with a manual and/or Z51 want to post what your settings are? We can see if GM is allowing for the different posted tire specs. And if not, perhaps the listed specs at Tire Rack are just wrong.
--Dan
Drifting
Joined: May 2006
Posts: 1,709
Likes: 18
From: Montgomery Il.
www.discounttire.com they have what you need. info center
this gets you really close
http://www.discounttire.com/dtcs/infoTireMath.dos
this gets you really close
http://www.discounttire.com/dtcs/infoTireMath.dos
Last edited by bosco 08; Dec 20, 2007 at 04:34 PM.