Knerf

the speedo in my car no longer works, cable and gauge seem fine, the issue that im seeing is after replacing the speedo driven gear in the tranny it works for about 10 miles and seems accurate, then starts jumping all over the place then nothing.

i have tried 2 different driven gears and a new bullet as the old one seemed sloppy, but same issue. can the drive gear in the transmission be the issue here or an i just missing something?

joewill

get under the car, disconnect the cable at the tranny and put a drill in the cable.. spin it and watch your speedo... this will tell you if your problem is in front of or behind your cable connection.
those drive and driven gears are plastic.. a binding cable could strip the plastic. the driven gear is 10$, the drive gear is on the rear shaft of the tranny requiring removing the housing to get to.. not fun or cheap

oledave60

I am right now going thru this same problem and I found out there are two different size drive gears and two different size driven gears. You must match the two are it won't work. It sounds like you have a small driven gear and a small drive gear.

Drive gears - 1.85" - purple, brown, natural, blue, red, silver

1.76" - green, black, yellow, orange

Good luck, Dave

Knerf

spinning the cable with a drill operates the speedo normally, i have 3.55 rear gears so went with the red driven gear as i thought was correct (21 tooth i think cant remember) maybe ill try one of the larger driven gears and see if that fixes it.

doorgunner

You should be able to turn the driven cable that goes up to the speedometer with your fingers and NOT feel any binding

While you have it apart.......use a bright flashlight to determine what color the DRIVE gear is on the tailshaft of the transmission.

ALSO.........lift the rear wheels off the ground/use jack stands....and rotate thedriveshaft slowly as you watch the drive gear to see if any teeth are chipped on it......also check to see if the "keeper" that holds the Drive gear in still in place.

73racevette

A sharp turn in the cable can cause the jumping around. Make sure your cable routing avoids sharp turns to the extent possible.

DUB

You are aware of the 'speedometer calculator' on the internet...if part of your issue it the speedometer is not reading correct.

YES...the drive gear can be a problem. I have seen them actually slip off the special clip and slide back so the drive gear does not contact it.

In my experiences..90+% of the time a bouncy speedometer is a bad cable...either binding or has been greased and the grease is heating up and causing the cable to 'pop' when turning....and as mentioned by '73racevette'...a shape radius causing the cable to bind.

DUB

Knerf

cable spins nice and easy with fingers, dont feel any binding there

pulled the gear out and the very ends of the gear are getting chewed up, like its not meshing fully because its too small. hopefully its the size of the drive gear that is the issue.

i will pull the holder out later today and take a look at that drive gear for damage though, really hoping its ok so i dont have to pull the tranny out.

wilesre

Isn't the drive gear steel? It is on my '70.

7T1vette

Later gears are both plastic. I believe the drive gear (on the shaft) is the harder of the two. As mentioned, the drive gear and driven gear MUST be compatible; there are tables that show which drive and driven gears work together for various transmissions. You can also use tables to select the proper gears for your car based on tires size and rear gear ratio.