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Borg Warner M12 Automatic speedometer drive gear

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71 Javelin AMX O.O. View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71 Javelin AMX O.O. Quote  Post ReplyReply Direct Link To This Post Topic: Borg Warner M12 Automatic speedometer drive gear
    Posted: Jan/07/2016 at 12:23pm
I was wondering if someone could check their M-12 Borg Warner automtic and let me know how many teeth are on the speeometer drive gear mounted on the tranny output shaft inside the rear housing?  The guys at PATC can supply different speedometer gears, but need to have 7 teeth on the gear in the transmission to work.  I was hoping somone might have theirs apart and could give me the teeth count.
 
Or maybe someone has the BW auto with 3.54 rear gears and has found a soultion to calibrating the speeometer with that rear axel ratio?
 
PATC can figure out how many teeth are needed on the cable gear, once they verify the teeth on the driven gear.
 
Steve
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sidewinder Quote  Post ReplyReply Direct Link To This Post Posted: Jan/07/2016 at 2:14pm
Try giving AMARK a call...he makes replacement gears and may know what you need. 

http://www.amarkamc.com/

Chuck Page


Chuck Page ,AKA Sidewinder
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amxdreamer Quote  Post ReplyReply Direct Link To This Post Posted: Jan/07/2016 at 3:54pm
The TSM lists all the gears for what rear ratio is used.
Tony
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71 Javelin AMX O.O. View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71 Javelin AMX O.O. Quote  Post ReplyReply Direct Link To This Post Posted: Jan/07/2016 at 4:19pm
Tony:
 
I'll check my Technical service manual tonight.  I did not realizre that info was there.  As far as you know the number of teeth on the inner gear or drive gear doesn't matter if you change the speedo cable gear?
 
The ones AMark lists are exacaly like the ones that Permormance Automatic Transmission show on their web site.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote scott Quote  Post ReplyReply Direct Link To This Post Posted: Jan/07/2016 at 4:51pm
There were 2 different drive gears, they differed according to the rear end ratio. The driven gear also differs according to the rear end ratio.

The M12 uses the same driven gear as just about any Ford application from the 60's into the 80's.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71 Javelin AMX O.O. Quote  Post ReplyReply Direct Link To This Post Posted: Jan/07/2016 at 4:59pm
Scott:
 
So if I give them the number of teeth on the driven gear they can match the one for the speedo cable for a 3.54 rear axel that I should be able to get from the Tech Ser Manual?
 
Have you ever changed out the driven gear?  I know I can get the rear housing off the tranny, but didn't check at that time how to remove the driven gear.
 
Steve
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Post Options Post Options   Thanks (0) Thanks(0)   Quote scott Quote  Post ReplyReply Direct Link To This Post Posted: Jan/07/2016 at 5:27pm
The best thing to do is drive your car at the MPH you want the speedo to be most accurate, let's say 60 MPH. Have a GPS in the car, & see how far off you are at that speed. Let's say speedo shows 60 MPH, GPS shows 72 MPH. That is roughly a 20% error, speedo reading slow. Pull your driven gear, count the teeth. Let's say it has 20 teeth. You need a gear with 20% less teeth to get the speedo close to accurate at 60 MPH. 20 X .8 = 16 teeth.

You do the opposite, add teeth, if the speedo is reading faster than actual.

If you can't get a driven gear that will correct the error, then you may need to change the drive gear. I've never had to do that, I guess I've been lucky.

I've heard that anything under a 5% error is considered very good. I know the truck I drive at work, a 2013, is about 10% off at 60 MPH, but it becomes slightly more accurate as the speed increases. maybe at 100 its dead accurate.

I'm not the greatest at math, so be sure to double check my calculations!!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71 Javelin AMX O.O. Quote  Post ReplyReply Direct Link To This Post Posted: Jan/07/2016 at 10:12pm
Scott:

I have 23 teeth on the speedo cable gear and I was told that the speedo was reading 10 mph under what I was actually going.  This was on a friends dyno.  As the speed increases the error is bigger.

So using your formula 23 x.8 would give me a speedo cable drive gear at 18.4.  AMark and PATC have 18, and 19 tooth gears.  I'm thinking try both and see what's the closest.  The guy at PATC said to verify if the driven gear on the output shaft has 7 teeth.  The best I could count with my finger was 6, but I'm thinking that it could be 7 as I couldn't really get a good feel.

Wish someone would have first hand info on this set up.  I can get the gear and install it on another cable and see if it spins without binding.  Then it should work on the road?

No specs in the factory TSM.

Steve
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amxdreamer Quote  Post ReplyReply Direct Link To This Post Posted: Jan/08/2016 at 3:47pm
In my 1970 TSM they are listed in a large chart covering pages 4-52 to 4-55.
 
Here is the section for 3.54 ratio.
 
Tony
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 71 Javelin AMX O.O. Quote  Post ReplyReply Direct Link To This Post Posted: Jan/08/2016 at 4:00pm
Tony:
 
What section did you find this chart?  I can look in my 1971 TSM again.  It didn't have any kind of this info in the transmission or rear axel sections.  I'm pretty sure that the driven gear has 7 teeth and the cable gear has 23.  This set up was for a go pac 401 autiomatic with A/C  I beleive the stock set up would have used a 3.15 ratio in the rear posi unit.  CATC said to get the speedometer to read more actuatly I would need a 20 tooth cable gear, which they are sending me.  He calulated it out by giving him the E-60 15 Polyglas tire diameter of 25".
 
Steve
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