Re-calibrating speedo
Thread Starter
Racer
Joined: Dec 2008
Posts: 327
Likes: 0
Re-calibrating speedo
I changed the rear-end in my '87 to a 3.07 and need to re-calibrate it. I found the parts #'s from the thread, but not all the parts are available. I need a new drive gear p/n 25513048 (41 tooth yellow), which is available, and a new speedo sensor p/n 25007308 (no longer available). I really need to know a direction to pursue. Does anyone have NOS, can I find them used? Any help would be greatly appreciated. Ed
Team Owner
Joined: Sep 2003
Posts: 25,377
Likes: 2,735
There are those that have mentioned "success" rotating the present speed sensor 180* to correct the "pitch" when using gears with tooth count of 40+. You may need to be "creative" when adapting the retaining device to work when it's rotated 180* but the theory should work. I have NEVER done the "rotation" personally!
I don't recall a 3.07 application that required a 41 tooth gear. Are you using an "online calculator" to determine that? I'm pretty confident a 3.07 generally requires a 42 tooth. Some/most online calculators use a tire diameter that you've supplied and to get an accurate speedo you need to work with "rolling radius"!!
The last 25007308 sensor that I bought came from the UK where the Carlton used the same ZF as the Corvette. To my knowledge there are no more there either.
You might want to try the WTB thing and see what you might land!!
I don't recall a 3.07 application that required a 41 tooth gear. Are you using an "online calculator" to determine that? I'm pretty confident a 3.07 generally requires a 42 tooth. Some/most online calculators use a tire diameter that you've supplied and to get an accurate speedo you need to work with "rolling radius"!!
The last 25007308 sensor that I bought came from the UK where the Carlton used the same ZF as the Corvette. To my knowledge there are no more there either.
You might want to try the WTB thing and see what you might land!!
Last edited by WVZR-1; Jun 6, 2009 at 08:09 AM.
Drifting
Joined: Oct 2006
Posts: 1,934
Likes: 3
From: Citrus Heights California
I changed my gears from 2.59 to 3.07 on my 88. I bought a 41 teeth gear and a new speed sensor from Patc Transmission.
The 41 teeth gear fit right into the stock speed sensor without any modification. It's been working OK for over a year. I was told that the OEM speed sensor would not work or that I could try to turn it around 180 degrees.
Another option is a Patc GM Ratio Adapter for around $80.00. They have the Speed sensor for $69.00. http://www.transmissioncenter.net/pe...automotive.htm The sensor and gears are near the bottom of this web page.
The 41 teeth gear fit right into the stock speed sensor without any modification. It's been working OK for over a year. I was told that the OEM speed sensor would not work or that I could try to turn it around 180 degrees.
Another option is a Patc GM Ratio Adapter for around $80.00. They have the Speed sensor for $69.00. http://www.transmissioncenter.net/pe...automotive.htm The sensor and gears are near the bottom of this web page.
Last edited by Kool88vette; Jun 11, 2009 at 11:13 AM.
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Melting Slicks
Joined: May 2001
Posts: 3,014
Likes: 18
From: Raleigh, NC
I have an electronic box that I bought from www.zfdoc.com that will correct for any rear end ratio. It was about $100 if my memory serves.
