replacing driven gear (speedo)

[rowingone]

My C2 has the wrong driven gear for the rearend that was put in . . . I just received the right one - any pointers on installation would be appreciated!

 

[JohnZ]

Disconnect the speedo cable from the "bullet", remove the 7/16"-head bolt and little retainer piece that hold the adapter "bullet" in the side of the tailhousing, and carefully withdraw the "bullet" from the hole, without dropping the plastic gear inside the housing.

Remove the old gear from the "bullet", lube the shank portion of the new gear and insert it in the "bullet", and reverse the above procedure. Check the O-ring on the bullet before you re-install it.

 

[allcoupedup]

Rowingone-
What are you changing from and what are you changing to? When I purchased my car, it too had the wrong driven gear. When I switched to the correct driven gear, it would not work.

It would not work because my transmission had the drive gear for 3.73 and up rear ends and my car had a 3.36:1. My point here is that if your switching from between driven gears for a low to high rear end ratio or vice versa, you're going to run into problems.

hope you don't have any issues but wanted you to be aware if you do.

Regards,
brian

 

[JohnZ]

Here's the skinny on midyear speedo drive and driven gears (from my "Tech Bench" column in the April '06 issue of "Corvette Enthusiast" magazine):

Here’s the story on speedo gears for ’63-’67 4-speeds and Powerglides.

Two different diameter 8-tooth steel speedo DRIVE gears were used, pressed on the output shaft, and each one used a different diameter “family” of nylon DRIVEN gears to accommodate a range of rear axle ratios. Transmissions for use with 4.56 axles used a special 6-tooth gear.

The “large” DRIVE gear was 1.85” in diameter, and used the “small” (0.8” diameter) family of DRIVEN gears, as follows:

3.08 = 18-tooth, #3987918 (brown)
3.36 = 20-tooth, #3987920 (blue)
3.55 = 21-tooth, #3987921 (red)
3.70 = 22-tooth, #3987922 (silver)

The “small” DRIVE gear was 1.77” in diameter, and used the “large” (0.87” diameter) family of DRIVEN gears, as follows:

3.70 = 22-tooth, #3860345 (green)
4.11 = 24-tooth, #3860347 (yellow)
4.56 = 20-tooth, #3860329 (steel – used with special transmission with 6- tooth 1.77” diameter DRIVE gear)

 

[rowingone]

Previous owner had switched to a 308 rear and not switched the driven gear so the speedo shows too low on the mph . . so I purchased the brown driven gear and thought that if I replaced whats in their with that I would solve the mph error - correct? thanks for the imput!

 

[rowingone]

Thanks to John Z I have taken out the driven gear per his instructions thinking I would find the incorrect one to replace with the correct one . . well the one in there was the brown one that is correct for a 308 rear that I was told the car had . . . well my speedo is off by 25-30% . . . possibly I dont have a 308 rear? .. could the speedo itself be off?

 

[JohnZ]

Highly unlikely that the speedo head is off - more likely that you have a 3.55 or 3.70 axle; what's stamped on the bottom of the diff between the spring mount plate and the camber strut rod bracket?

 

[rowingone]

John - O A O 64 as far as I can tell . ..

 

[rowingone]

John . . I"m assuming that the first letters must be "CA" which would be a 336 but the previous owner had drag raced it so I'm not sure if you can rely on the stampings as to what gears are in there . . . seems to me if I was only off one tooth on the driven gear I wouldnt see such a discrepency in the speedo reading . . .

 

[JohnZ]

Mark both tires and the prop shaft with tape, put it in neutral, turn both rear wheels one full turn, and count the revolutions of the prop shaft and that will tell you what ring and pinion are in it. A little over 3 is 3.08, 3-1/3 is a 3.36, 3-1/2 is a 3.55, 3-3/4 is a 3.70, and a little over 4 is a 4.11.

 

[rowingone]

thanks john 370 it is! I'll be ordering another driven gear . . . the right one this time! thanks as always for you help!