1976 Speedometer Error

[DanC]

My 76 had a transmission swap before I purchased it and the speedometer is registering very low. 25 mph indicated is closer to 55.

It has a "OK" differential, which, I think is from a 1978 model year, 3.08 ratio, according to the Corvette Parts Interchange Manual.

Can anyone advise what color code or number of teeth speedometer drive gear I need for this differential ratio? I haven't inspected the existing gear yet to see what's in there. Local Chev dealer was no help, says go to Eckler or Mid-America. My Mid-America catalog shows the color code and number of teeth but doesn't provide what ratio for what gear.

Thanks in advance,
DanC

 

[77-4speed]

DanC,

Remove your speedometer cable housing (the part that goes into the transmission) and count the teeth on the plastic gear that is attached to the speedo cable. That gear is the driven gear. As a general rule, every tooth you add, drops the speed by 5 mph. So if you're actually 30 mph too high, you need to add 6 teeth.

My local Chevy dealer had the speedo gears in stock for about $4 a piece. I tried one and it didn't work so I brought it back and tried another. The parts department at your largest local dealer should have the speedometer driven gear.

Russ

 

[Ala75Vette]

Let me echo 77-4speed's advice.... I recently had to do the same thing (swap out the driven gear). On my car I also have a speedometer coupling between the speedo cable and the driven gear housing that slips into the transmission. Once you remove the speedo cable and the coupling (makes a 90 degree turn for the cable), the driven gear housing has one retaining screw on the side to hold it in there. My coupling was also 'broke' so I wound up replacing it, too ($70), then the new one wasn't exactly 1:1, so I had to get a new driven gear with different teeth to compensate. Hardest part of the whole repair was getting the old coupling off the transmission.... you can see it just fine from under the car, but the exhaust is in the way of easily putting a wrench on it, and you have to be creative with 'viewing angles' to see what you're doing. Hopefully yours won't be as big a pain in the buttt as mine was... good luck!

 

[Dave L.]

speedo gear

I'll give you the part#,is it manual or auto

Dave

 

[DanC]

Burglar,

Thanks for the help. It's a 4 speed manual from a 79 (so I've been told).

Dan

 

[Dave L.]

correct gears for 3.08 axle & man trans.

The drive gear on the main shaft should have 8 teeth.If its the muncie trans. the part# is 6261784.If it's the borg-warner the part# is 6260705>this gear has been discontinued from GM.
The driven gear on both trans has 19 teeth and the part# is 3987919. It might be best to take 77- spds advice.See what tooth count is on your driven gear now.Let me know what you have and I'll let you know what gear counts are available for your trans. Worse case senerio would be having to go to gear adapter,hopefully we can avoid that. I hope this helps you out.

Dave

 

[DanC]

Thank you Dave. I'll get under it this weekend if the weather co-operates. I appreciate your interest.

Any suggestions to help determine what transmission I have?

Dan

 

[Dave L.]

Dan- The muncie trans has 7 bolts holding the side cover on.
The borg-warner has 6.Both cases are aluminum.

Dave