Speedometer Gear Reduction Units

[67 kemie ss]

I was looking up the threads on speedo gear reduction units and I had trouble interpreting what I found.  I have a 69Z with its original M21 and original 3.73 posi axle.  My question is, 'Do I need the reducer shown on UPC G80 sheet A2?  I'm thinking that I don't, but the guy putting my car back together says it's missing.  There was a question in an earlier thread as to whether hi-performance care with 15" wheels got the reducer.  I didn't see an answer to that. 

Thanks for any help. 

[JohnZ]

I was looking up the threads on speedo gear reduction units and I had trouble interpreting what I found.  I have a 69Z with its original M21 and original 3.73 posi axle.  My question is, 'Do I need the reducer shown on UPC G80 sheet A2?  I'm thinking that I don't, but the guy putting my car back together says it's missing.  There was a question in an earlier thread as to whether hi-performance care with 15" wheels got the reducer.  I didn't see an answer to that. 

Thanks for any help. 

As I recall, only the 4.56 and 4.88 axles needed the ratio adapter; other axles through 4.10 did NOT.

[67 kemie ss]

Thanks John!

[L78 steve]

Not 100% sure on this but like John stated 15" wheel cars no reducer up to and including 4:10s
14" wheel cars reducer used with 4:10 and up.
Would love to know for sure on this if anyone can comment. My 67 Camaro with 4:10s came without the reducer but was a racer and could have been pitched.

[KurtS]

There's two different trans versions with different speedo drive gears. One is for normal gears, one for deep gears.
IIRC, 4.10 is the crossover. It could use either trans, but one trans required the adapter when used with 4.10's. So it all depended on inventory and which trans version they used.

[JohnZ]

There's two different trans versions with different speedo drive gears. One is for normal gears, one for deep gears.
IIRC, 4.10 is the crossover. It could use either trans, but one trans required the adapter when used with 4.10's. So it all depended on inventory and which trans version they used.

My 02D '69Z was "born" with the 4.10 axle, and had the 25-tooth orange speedo DRIVEN gear (P9B17B, tag embossed 3946798). That's what leads me to the conclusion that the ratio adapter was only applicable to the 4.56 and 4.88 axles.

[L78 steve]

John I believe the Orange 25 is a later replacement. Mine has a natural 25.

[JohnKY]

Both 67 and 68 speedo charts list part #3860348 (orange) as the driven gear for Z28 w/4.10.
67 drive gear is #3708145 (steel)
68 drive gear is #3924097 (yellow)

I don't have a chart for 69, but can't think of any reason why they would change it.

Adapters only on Z28 w/4.56 or 4.88