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Diff gears vs speed vs rpm 

 

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 Post subject: Diff gears vs speed vs rpm
Posted: Sat Feb 12, 2011 12:35 am 
Oompa Loompa
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Joined: 30th Jan 2010

Ride: ef falcon

Location: brisbane
QLD, Australia

Hey guys I have a ef falcon with a au and a btr the diff is shagged so gone get it rebuilt I use it at the drags for the street series for a bit of fun it has a wade 977b in it and a 2400 convertor It I'm looking at 3.9's or even 4.11's but need to know wat the highway rpm will be at 110 I know it won't be great just need to know which will work beta as u have to drive to Rome for 1/8mile series
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 Post subject: Re: Diff gears vs speed vs rpm
Posted: Sat Feb 12, 2011 1:23 am 
Getting Side Ways
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 Post subject: Re: Diff gears vs speed vs rpm
Posted: Sat Feb 12, 2011 10:31 am 
Technical Contributor
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Ride: ED Falcon

Power: 133 rwkw

Location: Leeton
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My post from the different diff types explained forum..

gearbox-suspension-brake-driveline-f4/different-diff-types-explained-t59-140.html

Quote:
Ok...

Discovery... None of these take into account tire buldge (IE because it has weight on it)...

So I have 235 45 17 so to calc radius (I'll work in metric so it ends in km/hr)

45% of 235 = 105.75mm profile

17" = 431.8mm

So calc radius of my wheels are half of mag + profile = 431.8/2 + 105.75 = 321.65mm

Upon measuring (for whatever factors cause the difference) I measure 298mm

So that is 1 inch in radius and 2 in diameter that these equations are out...

If you use this formula to get km/hr (where * is times, / is divide, r is measured loaded radius from ground to wheel centre, R1 is transmission ratio, R2 final (diff) ratio, and RPM is your desired RPM, and remember order of operations (do whats in the brackets first))

(0.00037698 * RPM * r) / (R1 * R2) = km/hr

or to calc RPM when you know speed this way

(km/hr * R1 * R2) / (0.00037698 * r) = RPM

Example:

So in theory (still doesn't measure quite right though)

I think 5th is .63:1? and my diff is 3.27:1 at 100km/hr with 298mm radius


(100 * .63 * 3.27) / (0.00037698 * 298)

206.01 / .11234004

= 1833.81 RPM


Compare with the 322 calc'd measurement of the wheel is 1697.13, so 136rpm of difference. Not much but it makes it more accurate this way...

Hope it helps 8-)

 

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 Post subject: Re: Diff gears vs speed vs rpm
Posted: Sun Feb 13, 2011 2:03 am 
Getting Side Ways
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Ride: 93 ED sedan

Power: 161 rwkw

Location: Rockhampton
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this is how you work it out......remember RPM....3,800 = 3.8 .
Auto overdrive is 0.68 X ( diff ratio ) 3.9.....or....4.11....depending which way you go.

 

 

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