Speedo Gear ?

[Tom S.]

I changed the rear end from a 4:11 to a 3:36. Now I need to change the transmission speedo gear to match the 3:36 does any one know what the part# would be? Or how many teeth? I called the local speedo shop and he was no help at all. I tried my local dealer and they don't know. I'm sure i'm not the first person to do this!Thanks in advance.

Tom

[Brian Monticello]

Re: Speedo Gear ?

Tom,
Unfortunately, I think you'll need more than the speedo driven gear. You'll also need to change the speedo DRIVE gear which will entail removal of the transmission. You'll also have to remove the tailhousing and press off the circle drive gear....

3.36:1 needs the blue driven gear - 3987920

CAn't remember drive gear you'll need - assuming you need a steel gear

1.76" 3708145 8 tooth
1.84" 3978758 8 tooth

If you have a later muncie you'll have a nylon gear.

Brian

[Jim T.]

Re: Speedo Gear ?

3spee,4 speed, power glide, turbo 400, or tubo 350?

[Tom S.]

Re: Speedo Gear ?

Jim; Sorry I forgot the most important part. It is a M-21-4-speed. It is in a 69. I know what i'm working on so I thought everyone else did, again sorry for not including all the info.

Tom

[Brad K.]

Re: Speedo Gear ?

Try calling Ed Hartnett in Pennsylvania. (610-623-9381 or write him at eh4speeds@aol.com) He can fix you right up. He know's what he's talking about and was very helpful to me. You may or may not need a steel gear.
Brad

[Michael W.]

Re: Speedo Gear ?

Just have a look in your AIM, this will give you the correct combo of drive and driven gears.

[Brian Monticello]

Re: Speedo Gear ?

some good info:

http://chevelleengineer.home.comcast.net/speedo.htm

Brian

[Tom S.]

Re: Speedo Gear ?

Thanks for all the input. I have another question. Does any one make an adapter to do what I want to do. They used have an in line adapter on trucks that had 2 speed rear ends to correct the speedometer when you would shift from low to high range or visa versa. That would be a simple fix.Anyone have any ideas on this?

Tom

[Joe L.]

Re: Speedo Gear ?

Tom-----

For a 1969 with 4 speed, if you change from a 3.36:1 to a 4.11:1, you will have to change BOTH the DRIVE and the DRIVEN gears. The 3.36:1 uses a BLUE, nylon, 20 tooth driven gear of GM #3987920 in combination with a PALE GREEN, nylon 8 tooth DRIVE gear of GM #6261794. The 4.11:1 rear gear requires an ORANGE, nylon, 25 tooth DRIVEN gear in combination with a BLUE, nylon 8 tooth DRIVE gear of GM #6261795. The 6261795 is GM-discontinued but may be available aftermarket. All 1968-70 Corvettes utilize nylon DRIVE gears; all other year Corvettes with manual transmission or Powerglide utilize steel DRIVE gears.

You can, indeed, accomplish what you need to with the use of a speedometer adapter. This device modifies the output speed of the speedometer gear system by a fixed percentage and avoids the need to change DRIVE gears. For your application, you would change your speedometer DRIVEN gear to a 21 tooth, RED gear of GM #3987921. This gear will be compatible with your existing DRIVE gear and is an easy swap-out. Then, install speedometer adapter GM #1565208. Ultimately, you may need to change the speedometer DRIVEN gear up or down 1 tooth count to "fine tune" your calibration, but this will be an easy swap-out. If you need to go 1 tooth higher, then use a SILVER gear of GM #3987922; for 1 tooth lower install the BLUE 20 tooth gear you have now. All of these gears will be compatible with your existing DRIVE gear.

All of the above "goes out the window" if you're using tires that are of a significantly different OD than the original F-70-15 tire size. Also, the adapter is expensive. It currently GM lists for about 120 bucks.

[Duke W.]

Re: Speedo Gear ?

Any speedometer service station (look in your phone book under "speedometer") should be able to determine your speedo error and rig up a suitable Stewart-Warner adapter to bring the speedo within one to two percent accuracy.

Duke

[Dick W.]

Re: Speedo Gear ?

I have seen a lot of adapter kits from S.S. White at truck dealerships. They are fast becoming obsolete with the advent of electronic speedos.