I recently replaced my 3.08 rear end wih a 3.55 and now my crazy speedometer is off! No biggy, I expected this problem. But my question is how do I fix it and where do I get parts? I have the TH350 with stock tire size in my 78. The speedometer never has seemed correct and when I replaced the single cable with 2 new ones (going back hru the cruise control) the needle bounce got worse. So I re-lubricated the single cable and put it back on for now. Not much needle movement and no real bounce to speak of now.



Thanks in advance,
Rowdy

 

Rowdy, there is a small plastic speedometer drive gear that needs to be replaced (about $10). I believe, someone correct me if I'm wrong, it is behind the speedometer gauge itself. The gears differ depending upon whether your car is an auto or a manual shift. They also differ in the number of "teeth" in the gear. The auto gears range from 34 to 45 teeth, depending upon what your gearing is currently. You will need to ask the vendor which one is correct for the 3.55's. Try www.corvettecentral.com for the parts and info.

Kevin

 

You have several issues facing you! First off, the transducer for your CC may be the culprit for the needle bounce. Secondly, you can pull the speedo gear out of your tranny and replace with one intended for use with 3.55 gears. Your speedo now reads hig I would think - so you want a gear with less teeth.

I got these charts from Tom's '73 pages. They should get you started in picking a gear:

1973 Corvette Turbo-Hydramatic
Code Driven Gears Number of Teeth Sleeve Asm.
A 9780628 35T 1362294
B 1359272 38T 1362294
C 1362048 40T 1362293
D 1362049 42T 1362293



1973 Corvette Turbo-Hydramatic
Tire Size GR 70-15

3.08 Drive Gear 3.36 Drive Gear 3.56 Drive Gear 3.70 Drive Gear 4.11 Drive Gear

Driven A Adapt - Driven B Adapt - Driven A Adapt - Driven C Adapt - Driven D Adapt -

Unfortunately for you, it looks like like they specify a 35T driven gear for both 3.08 and 3.55 rear-ends. They call for use of an adaptor to correct the speedo. You will likely have to do the same. A good speedo shop (yes, they still exist in big towns - St Louis has one!) can likely sell you an in-line adaptor that will correct your speedo. Assuming your speedo was accurate BEFORE you changed the rear gears, 0.8676 gear reduction is what I get when I divide 3.08 by 3.55.

Good luck!

 

Go to www.tciauto.com click on technical instructions then click on speedometer gears. They have a calculator for determining the correct "Driven gear" you need based on which "Drive gear" you have already in the Transmission.
If you are using the 255/60/15 inch tires the correct diameter would be 27.1 inch. The gears are in the transmission and you can get at them by removing the speedo cable and the small housing. If memory serves me right the driven gear will come out and the drive gear will remain inside. Find out what colour the "Drive gear" is inside the transmission and put it into the formula to determine which "Driven gear" you need.
Good luck.

 

That's a cool little utility, THANKS!
So it sounds like I'll have to open the drive gear area up and see what colors are in there first though. I was kinda hoping someone would know what the starting (factory) setup colors for a 3.08 were so I'd only need do open it up once....no big deal.

Thanks again....

 

The gears inside the transducer appear to be particularly soft. Buying the whole transducer is fairly expensive, though you can occasionally find a usable one on EBay. If you're not using or can't use the cruise control, you're probably better off sticking with the single speedo cable, especially while sorting out the other issue.

 

To follow up on these issues:

If a muncie close ratio came with the gear reducer for the 4:11 rear. Can you just remove the gear reducer and then plug the cable right into the tranny?

If I understand this thread this would then be correct for a 3:55 right?

Sorry for following up a question with a question but this thread looks like the right time.

Thanks

 

My 1980 Corvette had the 8 teeth (orange) drive gear and the 19 teeth (natural) colour driven gear. It had the same diff. as yours 3.08. I did not have the optional wheels though (from the factory). My car came with the rally wheels. You will most likely have the (orange) 8 teeth Drive gear.

 

Correct me if I'm wrong but I thought the 1973 Corvettes had a turbo400. The speedo gearing for them is not the same as the turbo350.