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Old 12-20-2010, 07:44 PM   #1
rubrduk
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Default overall diameter questions - and how it affects gearing

the important specs:

daily driver...nothing crazy, track events on occasion, just for fun - not competition - i like driving my car hard in a safe place

current setup:
06' wagon
18x8 +45 wheels
225/40/18 tires

current overall diameter 225/40/18: 25.04"
stock overall diameter 215/45/17: 24.7"
tire i'm interested in 235/40/18: 25.4"


i'm in the market for new tires (mine are worn out), and a comment by a very knowledgeable and trustworthy wrx wagon owner at the recent AX practice day got me thinking

he said a 235/40/17 (24.45" OD average) was good not only for the tread width but was best for the gearing advantage

soooo....
this would require me to buy new wheels...and tires to gain said advantage


not being a math guy, the 1" OD difference between 235/40/17 @ 24.45" vs 235/40/18 @ 25.4" doesn't seem like much to me.

My question, will my butt dyno notice the big difference in gearing, or is it really a minimal difference for that extra .01 second at the track and not worth me spending the extra bones on for a complete rolling stock redo
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Old 12-20-2010, 09:47 PM   #2
rubrduk
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Default

bueller?
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Old 12-20-2010, 10:14 PM   #3
speedyHAM
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Your butt dyno might or might not feel any difference depending on how well calibrated it is. I noticed a difference between a 24" diameter wheel and tire and a 23.5" diameter wheel and tire that were very close to the same weight. Your autoX times will show a difference if you are a good driver and consistent in your lap times.
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Old 12-21-2010, 01:30 AM   #4
rubrduk
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Quote:
Originally Posted by speedyHAM View Post
Your butt dyno might or might not feel any difference depending on how well calibrated it is. I noticed a difference between a 24" diameter wheel and tire and a 23.5" diameter wheel and tire that were very close to the same weight. Your autoX times will show a difference if you are a good driver and consistent in your lap times.
i'm not the least bit consistent at autox, i'm still learning the basics
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Old 12-21-2010, 01:40 AM   #5
scooby24
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Try to put it in terms you can visualize.

Imagine you flipped a bicycle upside down and you're pedaling the wheel. Now make that wheel bigger. It's harder to get spinning but the outside of the wheel is spinning faster.

Now translate that to your car. It's going to take a bit more power to get that wheel moving. However, it'll lengthen out your gearing a touch.

In a DD situation, you won't notice a .36" increase in diameter.
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Old 12-21-2010, 04:13 AM   #6
rubrduk
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no here's a million dollar question,...

i know these are not NASIOC favorite tires, but i can get these in a 235/35/18 (24.6")

BFGoodrich Tires g-FORCE T/A KDW 235/35ZR18 90Y BW
Continental Tires Extreme Contact DW 235/35ZR18 86Y


does this do any good?
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Old 12-21-2010, 04:17 AM   #7
sc00by4life
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why would you want a sidewall that small? 235/40 is a much better option.
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Old 12-21-2010, 08:02 PM   #8
contour se driver
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Quote:
Originally Posted by sc00by4life View Post
why would you want a sidewall that small? 235/40 is a much better option.
most likely because stock is 17x7 with a 215-45-17 and the 235x35x18 is about dead on with the speedo with stock size.

i personally went with 245-40-17.
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