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Diff ratio 3.7:1 to 4.11:1 

 

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 Post subject: Diff ratio 3.7:1 to 4.11:1
Posted: Wed Oct 05, 2005 10:22 am 
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Hi guys

Whos running 3.7 ratio on their cars? I like to know what rpm you're sitting on at a 100 kmh on level freeway cruising.
I'm doing 1900 to 2000 rpm on overdrive at 100K's with 3.45:1 ratio.

got a quote for 4.11 crown & pinion for 250 to 300 bucks installed {in stock, secondhand} but I think its to high for freeway usage.

Just waiting for them to call back if they can source a 3.7 or 3.9s.

Should I go 4.11? :?

 

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Posted: Wed Oct 05, 2005 10:47 am 
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my car does 2050rpm @ 100 in 4th gear on the auto with a 3.45 diff.

3.7 would have it at 2200rpm @ 100
3.9 would have it at 2325rpm @ 100
4.1 would have it at 2450rpm @ 100

really not THAT bad... but then keep in mind you'll often do 110/120

 

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Posted: Wed Oct 05, 2005 11:14 am 
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just out of curiosity, what would are the RPM figures for 3.08, 3.23 and 3.45 diffs at 100km/h and 110km/h with an auto???

 

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Posted: Wed Oct 05, 2005 11:24 am 
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3.08 = 1830 @ 100
3.23 = 1920 @ 100

these are all as per my car.
my speedo is accurate to 1% (police spec cluster adjusted to suit tyres/diff)

 

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FALCN6 - EF GLi Turbo, 20" Rims, Air Bag Suspension, Straight LPG, 225rwkw
DTHWGN - EA GL Wagon, LTD Interior, Satin Black, 5.0, Turbo
RACER - EF Futura , 5.0, 5 Speed
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Posted: Wed Oct 05, 2005 11:46 am 
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EFFalcon wrote:
my car does 2050rpm @ 100 in 4th gear on the auto with a 3.45 diff.

3.7 would have it at 2200rpm @ 100
3.9 would have it at 2325rpm @ 100
4.1 would have it at 2450rpm @ 100

really not THAT bad... but then keep in mind you'll often do 110/120
I like the look of 3.7. Hold on, the 4.1 looks tempting since they got it on shelf. hehehe

thanks guys.

 

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Posted: Wed Oct 05, 2005 12:25 pm 
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How sure are you of those numbers? I know my wheels may be a different size but the differnce is massive in 4th 1:1 with 4.11 diff I sit on almost 3500rpm @ 100 thats almost 1000rpm difference to the numbers your giving there! think u may need to go back to the calculator.

 

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Posted: Wed Oct 05, 2005 12:34 pm 
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you just said it..
4th gear... mines an auto
4th gear = overdrive.

 

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Posted: Wed Oct 05, 2005 12:37 pm 
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MikGan wrote:
How sure are you of those numbers? I know my wheels may be a different size but the differnce is massive in 4th 1:1 with 4.11 diff I sit on almost 3500rpm @ 100 thats almost 1000rpm difference to the numbers your giving there! think u may need to go back to the calculator.
s**t!!!!!!!!!!!!!
looks like 4.11 is out of the question.

Whats 3.7 & 3.9 like then?

 

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Posted: Wed Oct 05, 2005 1:11 pm 
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EFFalcon wrote:
3.08 = 1830 @ 100
3.23 = 1920 @ 100

these are all as per my car.
my speedo is accurate to 1% (police spec cluster adjusted to suit tyres/diff)


thanks for that :)

 

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Posted: Wed Oct 05, 2005 1:32 pm 
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These are theoretical calculations:
4 Speed Auto - 4th gear 0.68:1
235/45/R17 tyres

3.08 = 1727rpm
3.23 = 1811rpm
3.45 = 1935rpm
3.70 = 2075rpm
3.90 = 2187rpm
4.11 = 2305rpm

a little different to what my car actually does :P
but its still a good estimate.

using this program:
http://locost7.info/gearcalc.php

 

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Posted: Wed Oct 05, 2005 2:58 pm 
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I had 4.11's in mine... you would be doing the same RPM you are doing now but 20K's slower.
I had to take off in 2nd sometimes... seroiusly not practical. and I don't go on the highway.. so around town it's not that great. Took off well though, traction is an issue.

Now with 3.45's. Heaps better.

Cheers

 

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Posted: Thu Oct 06, 2005 6:28 pm 
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i have 3:45's. XR diff. so i really had no choice.Happy with them though. But if i was to choose id go 3:70's :) Contact Preston diif's & auto's. St georges rd preston. Have a chat to them.

 

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Posted: Thu Oct 06, 2005 6:38 pm 
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not to steal the post. but i have a EA Ford Fairmont with a 3 speed. whats the diff on mine?

cheers
phong

 

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Posted: Thu Oct 06, 2005 6:43 pm 
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most likely a 2.77

 

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DTHWGN - EA GL Wagon, LTD Interior, Satin Black, 5.0, Turbo
RACER - EF Futura , 5.0, 5 Speed
PROJECT - 83 Thunderbird, Lowered, 18" Billet Wheels
DAILY - BA Fairmont Ghia 5.4 3V

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Posted: Thu Oct 06, 2005 6:52 pm 
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Ride: 07 BF mk2 E-gas wagon + eb 5.0

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for those with manual (t5) 5th is 0.72 so @ 100 in mine (3.45) it sits around 2150.

 

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