Ok I just picked up the 24 tooth gear and I am ready to put it in - but before I do that I have a question. The gear is definitely larger than the old 21 tooth gear - logically I am trying to wrap my head around how this meshes with the internal gear. Can someone say yes it is supposed to be that way and somewhere behind the case it all works? I fear plastic gear pieces floating throughout my transmission - I am sure I am just paranoid, but somebody reassure me

 

I've looked into the housing only twice....it's weird to see a "spiral cut" gear on the output shaft of the transmission facing rearward....and then install a "normal cut" gear "sideways" to mesh against the first gear.....BUT,IT WORKS....and you've double-checked to make sure the new gear is a "match" for the output shaft gear.

My concern is that I will have to buy a new DRIVE gear and install it if the speedometer is still too far off.........It's been about 5 years since I replaced a drive gear and 2 years since I replaced a driven gear.

 

Thanks! I now think I have the wrong gear.
From a thread I found it looks like I cannot replace a Red with a Yellow from the data below (large drive gear requires small driven gears and vice versa - so you must stay in the same families). Sounds like when someone replaced the rear gear ratio with a 4:?? they did not calculate you would really need to replace the drive gear also. Then they put larger tires on and multiplied the issue.

Two different diameter 8-tooth steel speedo DRIVE gears were used, pressed on the output shaft, and each one used a different diameter “family” of nylon DRIVEN gears to accommodate a range of rear axle ratios.

The “large” DRIVE gear was 1.85” in diameter, and used the “small” (0.8” diameter) family of DRIVEN gears, as follows:

3.08 = 18-tooth, #3987918 (brown)
3.36 = 20-tooth, #3987920 (blue)
3.55 = 21-tooth, #3987921 (red)
3.70 = 22-tooth, #3987922 (silver)

The “small” DRIVE gear was 1.77” in diameter, and used the “large” (0.87” diameter) family of DRIVEN gears, as follows:

3.70 = 22-tooth, #3860345 (green)
4.11 = 24-tooth, #3860347 (yellow)
4.56 = 20-tooth, #3860329 (steel – used with special transmission with 6-tooth
1.77” diameter DRIVE gear)

If you have the "small" drive gear on the output shaft and installed one of the "small" plastic driven gears in the adapter, it would result in exactly the symptom you described - speedo not working at all; only the "large" family of plastic driven gears will mesh with the "small" drive gear.

 

I think you will definitely need to use a chart like Willcox has......and you definitely will need to put a piece of tape on the drive shaft and on the wheel to count the exact amount of revolutions to determine the differential ratio in order to buy the correct "Family gear set".

I will know today if I have to do the same as you.

 

I installed a new driven gear tonight but will have to test drive the car tomorrow to check the speedometer reading.

 

The chart shows this gear match: DRIVE GEAR-20 teeth - green/DRIVEN GEAR-34 teeth - light green..........Apparently because of my 3.36 differential ratio and 60 Series tire diameter I am not getting the correct speedometer needle reading. (The differential is original, but the transmission was originally a 4-speed.

I finally tested the new driven gear..........I went from a 37 tooth to a 34 tooth.............WRONG......the speedometer now shows 140mph at 60 actual mph/previously it showed 80 mph at 60 actual mph



I also learned that the number of teeth on the DRIVE GEAR greatly affect the needle reading when the DRIVEN gear is changed. I read that most DRIVEN GEAR changes will cause the needle reading to change 3-5 mph.......In my case going from a 37 tooth gear to a 34 tooth gear with a GREEN DRIVE GEAR caused a 25 mph change with each tooth difference........your results probably won't be as drastic.

Now that I know I should have gone with a DRIVEN GEAR WITH MORE TEETH to "slow the needle down"...........and now I also know that with my CURRENT GREEN DRIVE GEAR I need to go to a DRIVEN GEAR that is only ONE tooth larger in order to get the approximate 20 mph needle drop that I need (the speedometer may be 5 mph "off", but I can live with that.

I have ordered a new 38 tooth driven gear.

I will update later.

 

Not sure if I am ready to put the larger yellow(24) gear in place of the red one(21) - the size diff concerns me and I do not want plastic pieces floating in my tranny!

 

It would be very useful if you knew whether a larger or smaller DRIVE gear is in the transmission now.

I can suggest that you temporarily remove the o-ring from the outside of the housing.....place the yellow gear into the housing.....slowly slide the assembly into the transmission while you "feel for any binding" as the drive gear and driven gear mesh together/if it feels as though the gears won't mesh---don't install the yellow gear.......if they mesh, take a small screwdriver......insert it into the driven gear square hole....slowwly turn the screwdriver to the right and then to the left to see how much/if any slack there is between the drive and driven gears to give you an idea of the clearance......place the transmission in nuetral and rotate the driveshaft by hand while watching the small screwdriver that is inserted into the driven gear to see if it rotates smoothly in either direction.

That's all I got!

 

Thanks I'll try it. The red smaller gear is currently in the housing. I guess if it don't fit smoothly don't force it. I still can't conceptually see how the larger gear fits against the metal interior gear the same as the smaller one - oh well guess I'll find out

 

I can only guess that the engineers decided how much slack the smallest driven gear could have and still be dependable.....it would be obvious to them how large the largest driven gear could be without binding.

For example: A small DRIVE gear and a small DRIVEN gear would never fully mesh together.....they would either "slip" or they would not mesh at all A large DRIVE gear and a large DRIVEN gear would fit too tightly against each other.

If you could put a paint-mark on one of the DRIVE gear teeth and slowly rotate the driveshaft while counting the teeth, you could determine which size DRIVE gear is already in the transmission......That would take a lot of guesswork out of the repair.