Crazy question. In a 4 or 5 speed trans, there is a plastic gear on the output shaft that drives the speedo cog. I need to know the number of teeth on that shaft gear.  I've had several trannies apart in the past and recycled more of these shafts that I can remember, but never kept one!

Why you ask? I have a electronic vehicle speed sensor on my 5 speed now, attached to the speedo cog output housing, and need to compute it's output frequency vs wheel speed. To get a ballpark number, I need the tooth count on the shaft gear to get the speed vs RPM data to enter into my Haltech tuning software. (I get my MPH from a GPS speedometer)

If someone just happens to have a tranny apart and can count, it will save me some time at the dyno.

Just to record this, I'm using a very special little dohicky made by a Toyota 4x4 offroad support company that came up with a lovely little low cost custom VSS that plugs directly onto the speedo cog housing of many Toyota's and quite conveniently many early Nissans's as well. Outputs the most perfect little 5v square wave. In case there are any others of you that are crazy enough to go DIY EFI in the future. Picture below. 

Edited August 11, 20195 yr by zKars

Side note, is it not interesting, how, given the number of speedo cog tooth counts there are out there for all the various diff ratio's, (yellow, blue, white, etc), that they all mesh properly with a single toothed gear on the transmission output shaft? Crazy how they made that work....

Or are all the different tooth count colored cogs different diameters as well, so the tooth spacing stays the same? The mysteries of the universe never end. 

 

Edited August 11, 20195 yr by zKars

https://www.classiczcars.com/forums/topic/62642-removing-280z-thttps://www.classiczcars.com/forums/topic/62642-removing-280z-tail-housing/ail-housing/

https://www.classiczcars.com/forums/topic/62642-removing-280z-tail-housing/

Edited August 11, 20195 yr by Zed Head

I do not have an answer for the teeth count, but I think you are correct as too all of the different tooth count colored cogs being different diameters as well .
I am sure they compensated for the different diameters by off-setting the housing to bring the gear back in full contact with the main gear.

Jim: Pulled the speedo gear out of a spare 4 speed A tranny to see if I could see anything. It is a worm gear with 6 "threads" from the start to finish.

Edited August 14, 20195 yr by 240dkw

Oh that's amazing! How did I not think of that? (One track mind, that's how...) Thanks for that!

Makes it a bit trickier to compute accurately with a worm. I don't think it matters how may turns of the screw the gear is made from, its got to do with the spacing or tooth angle or something too hard to compute or measure. 

Guess I'll have to do the work and turn a tranny and count tail shaft vs speedo rotations. Not like I don't have a few trannies laying around... Now if I can just remember what color gear I put in my tranny.... white I think. 

 

Ok, things make more sense now. I pulled my little collection of speedo pinions, and found out what's what.

The pinions are all the same diameter. What does vary, is the tooth count (we know that) and the ANGLE of the teeth across the gear. Fewer teeth, lower angle (compared to rotation axis). Pictures below show what I'm talking about.

And, sigh, I found one of the plastic output shaft gears in my stash... 

Yellow 3.9, blue 3.7, white 3.54.

And finally the answer to initial question, using the white 19 tooth 3.90 gear, 20 output shaft turns = 6.25 pinion turns. Now I have my VSS conversion. 

Interesting

Crazy, I look at the photo that I posted from ZParts and see different sizes. I would have never guessed that they are the same size.

zKars, your VSS will work perfectly for the speed input to the Electromotive TEC-GT ECU on my autocross car.

However, I hate to give up the OEM speedometer which I integrated into my dash. I'm looking for a pass-thru style sensor, something like that attached picture, but so far I've struck out.

Another thing I learned from this discussion is the thread size of 20mm x 1.50. Thanks.

 

You can also continue to use your speedo cable to drive your stock speedo and use a GPS based speed sensor to drive your ECU. 

I had an idea to build one of these last year, but I found out Haltech and ECU Master came out with one.  As long as you don’t auto cross in a parkage or tunnel, this could work.

https://www.haltech.com/product/ht-011310-gps-speed-input-module/ 

 

Edited March 19, 20205 yr by zKars

1 hour ago, zKars said:

 

You can also continue to use your speedo cable to drive your stock speedo and use a GPS based speed sensor to drive your ECU. 

I had an idea to build one of these last year

You just gave me an idea!

This is a GPS receiver sold by Adafruit. Adafruit Ultimate GPS

It is designed to be used with an antenna and an Arduino or similar mini-processor.

I already have the necessary parts and plan on using it for GPS tracking of the autocross courses. All I have to do is capture the speed signal and send it to my ECU. Easy!

Thanks for the inspiration.