Diff ratio is wrong?
Thread Starter
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Senior User
Joined: Mar 2007
Posts: 348
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Diff ratio is wrong?
Based on the door column stick:
I believe I have a 3.73 diff. ratio (axle code C1)
But hard numbers seem to arguing. At 65 MPH I turn about 2100 RPMs. Based on an RPM calculator Monsta pointed out:
http://www.angelfire.com/space/slr/RPMCalculator.htm
Using value of:
MPH = 65
Tire Diameter = 31.6 (I double checked this)
Axle Ratio = 3.73
Transmission High Gear Ratio = .71
I get an expected RPM of 1832
If I switch over to using
Axle Ratio = 4.3
I get an expected RPM of 2111 - much closer to what I'm seeing.
So which do I believe, my build sticker or the numbers from the calculator? Am I missing something? I'm going to be opening up the rear diff. on Friday - is there a sure way to tell by looking at the insides?
I believe I have a 3.73 diff. ratio (axle code C1)
But hard numbers seem to arguing. At 65 MPH I turn about 2100 RPMs. Based on an RPM calculator Monsta pointed out:
http://www.angelfire.com/space/slr/RPMCalculator.htm
Using value of:
MPH = 65
Tire Diameter = 31.6 (I double checked this)
Axle Ratio = 3.73
Transmission High Gear Ratio = .71
I get an expected RPM of 1832
If I switch over to using
Axle Ratio = 4.3
I get an expected RPM of 2111 - much closer to what I'm seeing.
So which do I believe, my build sticker or the numbers from the calculator? Am I missing something? I'm going to be opening up the rear diff. on Friday - is there a sure way to tell by looking at the insides?
Junior User
Joined: Jun 2006
Posts: 64
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Have you checked the tags on your differential?
Maybe you had some aftermarket modifications from a prior owner...I guess this would only apply if you aren't the original owner.
I'd be real curious to see what your diff. tags say.
I run 4.30's and at 65mph I'm up around 2100-2200 rpm. 70 mph is about 2350-2400 rpm.
Maybe you had some aftermarket modifications from a prior owner...I guess this would only apply if you aren't the original owner.
I'd be real curious to see what your diff. tags say.
I run 4.30's and at 65mph I'm up around 2100-2200 rpm. 70 mph is about 2350-2400 rpm.
Elder User
Joined: Mar 2005
Posts: 941
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Originally Posted by areed
I'd be real curious to see what your diff. tags say.
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Sit. Stay.
Joined: Oct 2001
Posts: 18,308
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From: Washington State
Originally Posted by phil6608
Sounds about right for a 3.73
I have 4.10s and I'm running 23 to 2400 RPMs at 70 MPH
I have 4.10s and I'm running 23 to 2400 RPMs at 70 MPH
~~~~~~~~~~~~~~~~~~~~~~~~~
I would bet on a wrong tag before I bet on a wrong ratio. Just to make sure you should get a few different calculations from that site and test 'em out. Like 40mph in 3rd (1:1) and other speeds in 3rd. I like third because it removes the a slipping TC from the equation. (Just make sure it is locked.)
Listen many things could be off. The speedo, the tach, the tag, etc. etc.
Honestly, the absolutely best way is to remove the cover and have a looksee inside.
Second best would be to jack up the rear and slowly spin the wheel while counting the driveshaft revolutions. A chalk mark on both makes it a lot easier.
Last edited by Monsta; Apr 3, 2007 at 12:13 PM.
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Thread Starter
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Senior User
Joined: Mar 2007
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Originally Posted by Monsta
Honestly, the absolutely best way is to remove the cover and have a looksee inside.
