formula for gearing??

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John Brown
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formula for gearing??

Post by John Brown » Mon Sep 18, 2017 12:22 pm

Does anyone know how to figure rpm/speed/ratio ?

I have a factory 3.90 and 25.5 in dia BFGs...

how much speed difference will a 3.70 make ?

is this formula achievable? or do I need more info?? I know I could maybe guess at the % difference.. but I need more solid numbers.

:?: :?
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Re: formula for gearing??

Post by DiaboJim » Mon Sep 18, 2017 12:44 pm

John

You need to have a RPM or Speed in MPH that you have in mind.
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Re: formula for gearing??

Post by disneyd » Mon Sep 18, 2017 12:45 pm

- Dave Disney

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John Brown
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Re: formula for gearing??

Post by John Brown » Mon Sep 18, 2017 12:48 pm

I think.. (not 100%) that Im revving out at 60 maybe.. I need to get to 70. (in second)

I was topped out yesterday at the trailer.. its killin me.. maybe I could have shifted. I went from 28.7 in factory 20s.. to 25.5 18s
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John Brown
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Re: formula for gearing??

Post by John Brown » Mon Sep 18, 2017 12:50 pm

disneyd wrote:
Mon Sep 18, 2017 12:45 pm
Will this help?

http://www.apexgarage.com/tech/gear_ratios.shtml
awesome Dave! I think that will help me
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John Brown
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Re: formula for gearing??

Post by John Brown » Mon Sep 18, 2017 1:10 pm

Well, that sounds accurate. at my factory limiter of 6500rpm.. Im topping out at 60 MPH.(my STS car was faster..lol) If I swap to a 3.70... I only get 3 MPH. but if I can up the RPMs to 7K.. that nets me 68+ which is real close to what IM looking for.

That calculator works great Dave.... Thanks
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Re: formula for gearing??

Post by thrdeye » Mon Sep 18, 2017 1:37 pm

John, you could also think about going the other way. You have a lot of power and torque, so maybe think about getting into 3rd sooner but being able to hold it? Could make the car easier to get on throttle.
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Re: formula for gearing??

Post by jcox07 » Mon Sep 18, 2017 1:38 pm

Third gear.... whats third gear...lol
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Re: formula for gearing??

Post by John Brown » Mon Sep 18, 2017 1:44 pm

I almost started in 2nd.. (more than enuff) and use 3rd. Thats my mission in Bristol.

Im just not fluid in the car yet to try things.. but, I will.
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Re: formula for gearing??

Post by thrdeye » Mon Sep 18, 2017 1:46 pm

4.10!
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Re: formula for gearing??

Post by jcox07 » Mon Sep 18, 2017 1:54 pm

I actually started in second on all my runs....
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John Brown
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Re: formula for gearing??

Post by John Brown » Mon Sep 18, 2017 2:04 pm

dammitt....
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Re: formula for gearing??

Post by Challenger » Mon Sep 18, 2017 2:21 pm

John Brown wrote:I almost started in 2nd.. (more than enuff) and use 3rd. Thats my mission in Bristol.

Im just not fluid in the car yet to try things.. but, I will.
I did start in 2nd my last run....didn't work out too well...lol. I threw my mental timing all off in my head and missed my braking point going into the turn around and screwed up the corner. But it could have worked because it didn't bog the car or anything.

I've gone to 3rd a couple times this year and tried leaving it there once when we ran the Bristol drag strip. Worked out good till the finish slalom. I just felt like I didn't have any acceleration getting into the finish. But with even shorter tires that may work out. 2nd & 3rd may be the key.

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John Brown
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Re: formula for gearing??

Post by John Brown » Mon Sep 18, 2017 3:04 pm

if this works... the useful RPM range would be 3000 - 5000. 5K is 67-68 MPH

with 85 MPH being the limit that we will not hit. Steve, do you think it has usable power at 3K ??
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Re: formula for gearing??

Post by Challenger » Mon Sep 18, 2017 3:50 pm

I think 3K may be a tad low. I've noticed it really doesn't start pulling hard until somewhere around 3500-4000. But we're not talking much there so I think we may lose less that way than falling flat on the limiter for a second or so.
What if we were to go the other way with a 4.10 diff?

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