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Speedometer gear

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#1 sredmyer


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  • Legal Name:Steve Redmyer

Posted 11 November 2017 - 12:27 PM

So I just bought another 70 SS454 which had the rear gears changed to 3:55s.  While I like this gear a lot for local cruising (and tire spinning of course) it is not so good on the freeway.  That said, I think I will leave it as is at least for now.  Can anyone tell me if I can simply change the speedo gear in the transmission to correct the is currently reading about 90 MPH when doing 70 on the freeway.


1970 Monte Carlo SS454 Triple Black, Numbers Matching
2013 Cadillac XTS Platinum
2011 Ford Super Duty (F350) King Ranch

#2 stangeba


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Posted 11 November 2017 - 03:16 PM

I think so. Here is a web site that may help.


Posted Image
Original Owner since May 14 1971



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Posted 11 November 2017 - 10:34 PM

Here is another Calculator Tool to help you .  If you know what your Drive & Driven gears in Transmission are, it will help in deciding what course of action to take.  I had the same issue when I replaced the Rear End & gears to 331:1 ratio 12 Bolt Posi from a  273:1 ratio 10 Bolt if I remember right.  I ended up putting a Gear Reduction devise between speedo cable at Tranny connected to devise and to Transmission.  Damned if I can remember what the Technical name is.  I was way off and brought actual speed within 1% of accuracy. 

Pretty sure you will get a lot of help on this.  :)     






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1972 CUSTOM (Z03) - 402 CI, TH400, 12 Bolt 331:1 Posi, Power Driver 4-way Seat, Console, Full Gauges, Tilt, A/C, AM/FM Stereo, Power Trunk, Windows, Locks, Cruise Control & Keyless Entry

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