General chat that fellow COM'ers may be interested in.
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John F
- Speed Racer

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by John F » Fri Feb 10, 2012 2:07 pm
You going to get a pair of the 315/35R17's for the front Mark?
John F
#167 SA FFR roadster
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breakaway500
- Speed Racer

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by breakaway500 » Fri Feb 10, 2012 3:24 pm
I went with the 245's for the front of the Focus,and skis for the back..

It's not what you drive, it's how you drive. "Lap times matter"
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rousespointer
- Rookie Racer

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by rousespointer » Tue Feb 14, 2012 9:28 am
Is a Hoosier size 285/645R18 close to a size 285/30ZR18? I can't remember how to convert the numbers? What does the -O mean on the list?
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dradernh
- Speed Racer

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by dradernh » Tue Feb 14, 2012 11:20 am
rousespointer wrote:Is a Hoosier size 285/645R18 close to a size 285/30ZR18? I can't remember how to convert the numbers? What does the -O mean on the list?
The 645 tells you the diameter of the tire, which is 25.4" (645mm / 25.4mm per inch = 25.4"). I don't know exactly how to calculate the aspect ratio from that data, but a call to Hoosier will get you squared-away in no time. My guess is that it's between 30 & 35.
'95 M3 LTW #283 SB
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Chrispy
- Speed Racer

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by Chrispy » Tue Feb 14, 2012 5:21 pm
dradernh wrote:rousespointer wrote:Is a Hoosier size 285/645R18 close to a size 285/30ZR18? I can't remember how to convert the numbers? What does the -O mean on the list?
The 645 tells you the diameter of the tire, which is 25.4" (645mm / 25.4mm per inch = 25.4"). I don't know exactly how to calculate the aspect ratio from that data, but a call to Hoosier will get you squared-away in no time. My guess is that it's between 30 & 35.
The math is really ugly in the tire world thanks to the combination of metric and imperial measurements
But by my calculations the profile of 285/645R18 would be 33, so in the tire world it could be rated as either a 30 or a 35 profile.
A = ((Di - Ri) x 25.4) / (Wm x 2)
Where:
A = Aspect ratio
Di = Diameter in inches
Ri = Wheel radius in inches
Wm = Section width in mm
To go the other way:
Di = (Wm x A x 2 / 25.4) + R
Last edited by
Chrispy on Tue Feb 14, 2012 9:07 pm, edited 1 time in total.
Chris Parsons
#22 - 95 Miata
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breakaway500
- Speed Racer

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by breakaway500 » Tue Feb 14, 2012 7:44 pm
^Very impressive,Chris!

It's not what you drive, it's how you drive. "Lap times matter"
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Chrispy
- Speed Racer

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by Chrispy » Tue Feb 14, 2012 9:06 pm
Yep, 9th grade alegbra, LOL.
Wish I could remember the years that followed, I hear they taught us some cool stuff.
Chris Parsons
#22 - 95 Miata
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CP
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by CP » Tue Feb 14, 2012 11:36 pm
225-245/45/R17 "Continental Grand Am Series Tires" for $147ea...what a damn DEAL! Ack, if only I could run these on my street car!
-Cy
99 Spec Miata (SM/STU/STL/EP)
2011, 2013, 2014 NER STU Champion
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