Speedometer gear question

[vetteboy79]

Hello -

My speedometer hasn't been working for some time now...so I finally got around to troubleshooting it. The speedometer itself is mechanically sound (the needle goes around when i put a drill on it)...however the speedometer cable only turns when the car is in reverse. Not forwards. I think the cable is ok (i've got a replacement one anyway) so I'm wondering if it's the speedometer gear in the tranny. Problem is, in Corvette America they've got about 8 different speedometer gears for the C3 vettes, differentiated by color and number of teeth...I guess this is for all the different axle ratios and such. Can anyone tell me what the proper speedo gear is for a '79 L-82 4-speed with a 3.73 rear?

Thanks!

P.S. Before anyone asks...I don't want to just go by the one that's already in there, because it's possible that the previous owner put the wrong one in. More and more I'm finding that the last owner of the car had no clue what he was doing...

 

[JohnZ]

Your local Chevy dealer parts guy should be able to tell you what speedo driven gear you need, but he won't find a '79 with a 3.73 gear in his tables - have him look up a 3.70 instead, and that gear will be as close as you'll get. Also depends on your tire size, but I'm assuming yours are close to stock overall diameter.

 

[vetteboy79]

thanks

Ooops - I meant to say 3.70 for the axle. The drivetrain is pretty much stock in all respects. Thanks!

 

[Eddie 70]

Are you saying that the needle only moves when you put the drill in reverse. If so that is normal. Next time you are under the car stick your finger in the hole where you pulled the gear out of and see if you can feel the gear that is on the output shaft of the tranny. This is what happened to mine.


When you run the drill in reverse it is the same as driving the car forward. When the drill is in forward it is the same as driving in reverse. The C3s are not like the newer cars and accumulate mileage while going backwards.

 

[vetteboy79]

clarification

Eddie70-

Here's what I meant...when I drive the car forwards with the speedometer cable attached properly to the trans, the cable doesn't turn. When I drive it in reverse, the speedometer cable does turn. The net effect of this has been that my car is now showing slightly less mileage than when I got it, because it only spins the speedometer when the car is going backwards. However, if I put a drill on the end of the speedo cable, I can make the needle go up, which is why i'm thinkin it's more the gear than the cable. Like I said, the previous owner of the car was a dumbass who probably broke more things than he fixed, so I've got no clue if it's even set up right. I'd like to get this fixed soon, though...I'm getting tired of estimating speed from the tach reading

chris

 

[Eddie 70]

One more thing, does your car have cruise control on it. Look and see if the cable is fully screwed in at the cruise control module.

 

[LanceB]

My cruise control cable from the carb to servo is shot. Can the speedometer still work on a 77 without the cruise working?

 

[LanceB]

Here is where I found out what speedometer gears I am going to need, but I haven't ordered them yet. Vetteboy79 I don't know what is going on with yours but I am changing my speedo gears soon, so if you figure it out let me know what info you have. There are a lot of posts on this, must be a common problem.


http://www.bowtieoverdrives.com/index.shtml

 

[JohnZ]

My cruise control cable from the carb to servo is shot. Can the speedometer still work on a 77 without the cruise working?

Yes, the speedo will work regardless of the servo-to-carb cable. BTW, the post you responded to is over four years old.