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Borg Warner M12 Automatic speedometer drive gear

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scott View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote scott Quote  Post ReplyReply Direct Link To This Post Posted: Jan/08/2016 at 5:20pm
The reason it is best to calculate the error at a set speed rather than using what a chart says is correct is this: The speedometer head itself may not be 100 % accurate. Speedometers are supposed to run exactly 60 MPH at 1000 RPM. And that is what the charts are based off of. However, there is a lot of room for error in the speedo head. The spring could be weak, & make it run fast. Magnets could be weak & make it run slow. And vice versa.

If you calculate the error while running down the road at a set speed, this compensates for any error in the speedo head itself. It also allows for converter slippage at that MPH.

My calculation above was just an example. To come up with a number to multiply your 23 tooth gear by, we need to know what indicated MPH it was 10 actual MPH slow. However, let's say it was at 60 MPH. If that is the case, the calculation should get you close. Probably the 19 tooth will get you close to accurate.
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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 6:41pm
Scott, what about the guys that say their cars run 145mph with their GPS with small tires and basic powertrain. Mathematically impossible.
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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 6:48pm
Originally posted by 71 Javelin AMX O.O. 71 Javelin AMX O.O. wrote:

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
I looked for a while to find it, its at the back of the electrical section. When I was in Cleveland I was looking for a gear for my M11B because I had switched to a different rear ratio (3.31). I looked through a manual for a long time to find it and then another guy there finally found it for me.
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/14/2016 at 1:01pm

I got the 20 tooth speedo drive gear from CATC and will see how it fits and compares to the 23 tooth one that is running now.  I was thinking of jacking up the back and hand turning the driveshaft and see if the gear has no binding issues before I take it on the road.  I still have not gotten a visual or confirmation from anybody if the driven gear on the trans output shaft is a 7 tooth.  I can't quite tell when I feel for mine.

 
Steve
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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: Feb/11/2016 at 4:00pm
Well I didn't like the longer end pin on the CATC 20 tooth gear, so I got the AMark one and it looked a lot better.  I put some light grease on the teeth and installed it in the tranny and hand turned the rear wheels.  I pulled the gear and could see that the teeth were meshing and making contact at the right place.  I'll find out this spring, once we get a decent day here in Minnesota, if it's recording a more accurate MPH on the speedo.
 
Steve
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Post Options Post Options   Thanks (0) Thanks(0)   Quote vyscera Quote  Post ReplyReply Direct Link To This Post Posted: Nov/13/2017 at 5:45pm
How did it work out? I'm missing the speedo gear and am trying to figure out where to start for my Machine. 
Only One is a wanderer, two together are always going some place...
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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: Nov/14/2017 at 10:27am
vyscera:
Great. It reads a lot more accuatly as was confirm with my I phone speed box app. I can send you some pictures of the installation. I had to replace the driven gear on the output shaft of the Borg warner M12. Just replacing the speedometer gear was not making any difference.
Steve
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