I found out why my speedo isnt working. There is a l shped assemble that connects the cable to the tranny that has two gears in it. The gears no longer mesh. I found the part at ecklers for about $50 but i dont really want to spend that much. Is there any way to get an adapter to bypass this piece i dont think it is original anyway or is there anywhere else that sell it cheaper.
Thanks

 

sorry i didn't check for typos before i posted.

 

It sounds like an adapter someone had put on to correct a speedometer inaccuracy, probably from changing a rear end ratio or putting on different profile tires. You can get the proper driven gear for a muncie from most places for about 5 bucks. It depends on your tire size and rear gear ratio. My car has a 3.70 rear and 255/60R15 tires and I use a 20 tooth blue gear, it's pretty accurate. My Camaro had a 3.73 Rear and 235/60R15 tires and it uses a 22 tooth silver driven gear.

I would suggest throwing that adapter out and getting the correct driven gear. It takes less than a minute to change. You loosen the 7/16 screw, flip the tab to the side, and pull the piece straight out. the gear just comes out. The gear and the housing are standard TH-350 parts and are easily available just about anywhere.

It may not be that simple though. There are two different drive gears, and one of them is used with 18-22 teeh gears, and the other is used with 22-25 tooth gears. For instance, if you have a 19 tooth gear, but need a 23 tooth gear, you'll have to take the tailhousing off of the tranny, and replace the drive gear. Not too terrible, but it will be a real bitch to do with the tranny in the car. I'm not even sure you can do it in the car on the vette. I've done it on other cars, but I recommend just pulling the tranny out of the car, it's less hassle.

Post your tire size and rear gear ratio, maybe we can get you close to the right gear size. Find out what gear you have in it now also. Specifically the Color and number of teeth.

Joe Lucia made an excellent reply to one of my speedo questions about 3 weeks ago. In it he listed pretty much every gear and it's associated color. I'd do a search for it.

 

Justin------


The device that you describe is a speedometer adapter. Your car does NOT require one PROVIDING that

1) Your tires are at or near the same diameter as the original tires (27.0")

2) Your transmission is the original (or has the same internal speedo DRIVE gear installed as original)

If you provide information with respect to what your rear end ratio is and what speedometer DRIVEN gear is currently installed in your transmission, I'll be able to tell you what DRIVEN gear that you require with NO adapter installed.

Keep in mind, however, if your tire diameter is markedly different from the original you will likely require an adapter unless you wish to change your transmission's INTERNAL speedometer DROVE gear. In that case, it's usually easier and less expensive to just use an adapter.

 

I have 3:55 rear gears and a munchie M20 with 245 60 15s . Thanks for all the help. :cheers:

 

Justin------


The P245-60-15 tire size has a diameter which is very close to the original tires. So, this should not be a factor in any need for a speedometer adapter.

Your car should use a 22 tooth green gear of GM #3860345. That should produce an accurate speedometer reading without an adapter. This is how your car was originally equipped.

If the gear that you remove from your transmission is, for any reason, a 19 tooth "natural" gear, a 20 tooth blue gear, or a 21 tooth red gear, then, because of DRIVE gear incompatibility, you will not be able to use the 22 tooth geen gear. If that is the case, use the 22 tooth silver gear of GM #3987922.

 

Joe, what do you know about 700r4 speedo gears? I think I'll need an adapter with my 4.56, but what driven gears are available?

 

Had the adpater fail on my '75 as well. The assembly manual for your car will also tell you what gear you have/need as well as an adapter.
The adapter I had was a .88:1 ratio which explained my speedo error. I replaced it the one from Corvette Central that was 1:1.
Why an adapter? Well, with my custom bent dual exhaust, it was easier than trying to bend the cable 90 degrees. Also has a grease fitting.

Good diagnosis though.

 

"Racer16k"------


For Turbo-Hydramatics (350, 400, or 700R) speedometer DRIVEN gears of 34 teeth through 45 teeth are available. However, the highest tooth count gear, the 45 tooth light blue GM #9775187, will not provide an accurate speedo calibration. So, an adapter will be required.

With tires equivalent in diameter to the stock GR-70-15, I THINK that the following combination will provide a satisfactory calibration. However, I'm no guaranteeing this and your best bet would be to have a speedometer shop calibrate it. In any event, here's what I come up with:

speedometer DRIVEN gear-------- GM #1362048 (40 tooth black gear)

speedometer adapter-------------GM #3932220 (0.7333:1)

 

Thanks Joe,

That should get me in the ballpark. I can't even see the park right now if you know what I mean.

 

Thanks for all the help the gear was a blue gear. I just bypassed the adapter and with the blue gear it reads 59 when with the adapter it read 55 so i think i will just leave it for now. Maybe someday i will change it out when im not so lazy. Thanks again. :cheers: