Borg Warner M12 Automatic speedometer drive gear
Printed From: TheAMCForum.com
Category: The Garage
Forum Name: Transmission & Drivetrain
Forum Description: If it's between the engine and wheels, it goes here
URL: https://theamcforum.com/forum/forum_posts.asp?TID=76128
Printed Date: Apr/19/2024 at 2:07am
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Topic: Borg Warner M12 Automatic speedometer drive gear
Posted By: 71 Javelin AMX O.O.
Subject: Borg Warner M12 Automatic speedometer drive gear
Date Posted: Jan/07/2016 at 11:23am
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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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Replies:
Posted By: sidewinder
Date Posted: Jan/07/2016 at 1:14pm
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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 1968 "BLACK CHERRY" Javelin Previous owner of 2013 Heritage Cup winner 1970 BBO AMX |
Posted By: amxdreamer
Date Posted: Jan/07/2016 at 2:54pm
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The TSM lists all the gears for what rear ratio is used. ------------- Tony Vancouver, BC 1970 AMX 1972 Badassador AMO#10333 |
Posted By: 71 Javelin AMX O.O.
Date Posted: Jan/07/2016 at 3:19pm
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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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Posted By: scott
Date Posted: Jan/07/2016 at 3:51pm
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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. |
Posted By: 71 Javelin AMX O.O.
Date Posted: Jan/07/2016 at 3:59pm
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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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Posted By: scott
Date Posted: Jan/07/2016 at 4:27pm
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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!! |
Posted By: 71 Javelin AMX O.O.
Date Posted: Jan/07/2016 at 9:12pm
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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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Posted By: amxdreamer
Date Posted: Jan/08/2016 at 2:47pm
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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.
http://s182.photobucket.com/user/amxdreamer2/media/misc%20car%20parts/20160108_124237_zpsklcsjzn4.jpg.html" rel="nofollow">  ------------- Tony Vancouver, BC 1970 AMX 1972 Badassador AMO#10333 |
Posted By: 71 Javelin AMX O.O.
Date Posted: Jan/08/2016 at 3:00pm
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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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Posted By: scott
Date Posted: Jan/08/2016 at 4:20pm
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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. |
Posted By: amxdreamer
Date Posted: Jan/08/2016 at 5:41pm
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Scott, what about the guys that say their cars run 145mph with their GPS with small tires and basic powertrain. Mathematically impossible. ------------- Tony Vancouver, BC 1970 AMX 1972 Badassador AMO#10333 |
Posted By: amxdreamer
Date Posted: Jan/08/2016 at 5:48pm
------------- Tony Vancouver, BC 1970 AMX 1972 Badassador AMO#10333 |
71 Javelin AMX O.O. wrote:Posted By: 71 Javelin AMX O.O.
Date Posted: Jan/14/2016 at 12:01pm
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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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Posted By: 71 Javelin AMX O.O.
Date Posted: Feb/11/2016 at 3:00pm
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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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Posted By: vyscera
Date Posted: Nov/13/2017 at 4:45pm
| How did it work out? I'm missing the speedo gear and am trying to figure out where to start for my Machine. |
Posted By: 71 Javelin AMX O.O.
Date Posted: Nov/14/2017 at 9:27am
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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 |
Posted By: roadventure
Date Posted: Aug/04/2023 at 3:05pm
According to the TSM (page 4-48) the BW M12 transmission has a 9 tooth speedometer DRIVER gear. This is true for the 2.87, 3.15 rear gear ratio. It is unlikely someone would have changed the speedometer DRIVER gear if the rear drive ratio is changed, so it is very likely that your car has the 9 tooth DRIVER in the M12 transmission.
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