Author Topic: correct speedo gear  (Read 6347 times)

Tinkerr

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correct speedo gear
« on: July 07, 2006, 07:31:56 PM »
I'm changing the gear ratio from 3:08's to 3:73's and would like to change the speedo gear  to reflect the correct mph. Does any body have a chart that shows what color/tooth count coincides with what. The turbo 400 originally had yellow/41 tooth gear.The car is in the final stages of paint,then reassembly begins. I'd like to address this now and have the trans portion buttoned up. Thanks

Ed B

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Re: correct speedo gear
« Reply #1 on: July 07, 2006, 09:29:28 PM »
Does THIS CHART help?

Ed

lakeholme

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Re: correct speedo gear
« Reply #2 on: July 08, 2006, 09:42:16 AM »
If it didn't help Tinkerr, it sure helped several of the rest of us.  We've been hunting for that chart.  Thanks, Ed!

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Tinkerr

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Re: correct speedo gear
« Reply #3 on: July 09, 2006, 03:12:48 AM »
Ed.
It certainly will help in the future, but right now I don't have tires. I had hoped to resolve this before rolling the completed sub-frame under the painted shell. My thought was that GM used a chart that stated 14" wheels with turbo 400 = X color/tooth count gear, 15"tires with manual = X and so on. They must have had some reference, I'm sure it wasn't trial and error when deciding what speedo gear went with what combo. Having said that 40yrs. after the fact we may never know. Thanks   

Ed B

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Re: correct speedo gear
« Reply #4 on: July 09, 2006, 06:11:59 AM »
Tinkerr,

Yes, there is a chart in the P&A, but you need to know the transmission, rear axle ratio and the exact tire size. We know all but the tire size. Without that you can't work out the speedo "drive" or "driven" gears. Here's the 69 Chart to help. If you can give me the part numbers of the two gears ("driven" and "drive"), I can tell you which one's to get.

Ed


Tinkerr

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Re: correct speedo gear
« Reply #5 on: July 09, 2006, 10:23:41 PM »
Ed,
Tire size would be F-70-14". I believe that to be the original tire size(68 396/325 RS/SS turbo 400 3:73 r/p). In looking at the chart the correct #'s appear to be 8623311 drive/9774413 driven. I noticed that the speedo adapter comes into play at that pt#, any idea what that means? Additionally the chart shows standard 396 then hi/p  sp.hi/p. The 396/325 would be standard performance, but 3:73's arn't mentioned.How does hp. play into this, the key should be the trans. with hp. having no bearing on the speedo. Odds are good I'm overlooking something. Thanks Paul

Ed B

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Re: correct speedo gear
« Reply #6 on: July 10, 2006, 12:52:12 AM »
Tinkerr,

I'm not up on 69's as much as 68's, so what I write here is only a guess. 69 experts, please fix my mistakes!

I believe the lowest axle ratio you could get with the 396/325 and the M40 (Turbo 400) was 3.31. If you wanted lower gears than that, I think you had to upgrade to the heavy duty (or "High Speed") Turbo 400 that was standard on the 396/350 and 396/375 cars. This was a different transmission with heavy duty parts (see the product bulletin attached).

Because there were only so many combinations available for speedometer gears (due to physical size and teeth count), very low axles used a speedometer adapter to convert the output of the transmission speedometer gear. For example... Notice in the chart above that all heavy duty turbo 400's use the same 8623311 drive gear, and that the 2.73 axle and 3.73 axle use the same 9774413 driven gear. How can this be? If you used this combination in the 3.73 axle car, the speedometer will be WAY off!! Howver, by the use of a reducer (adapter) for the 3.73 axles you can use the same combination and still get accurate speedometer readings. Boy, those Chevy engineers were sharp, eh?

So in your case, with a Turbo 400, F70-14 tires and a 3.37 axle, you're going to need a 16 tooth (orange) "Drive" gear, a 34 tooth (Light Green) "Driven" gear, and a 3932220 adapter (reducer). That's going to be the hard part, finding the adapter. They come up on ebay periodically, so check often.

Hope this helps, and like I said, if I made any mistakes (which is VERY possible), some of the 69 guys will correct me.

Ed


Tinkerr

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Re: correct speedo gear
« Reply #7 on: July 10, 2006, 03:47:40 AM »
Ed,
I was looking in Rick's catalog at speedo parts and see they only list 1 driver (press fit) no tooth count. That would lead me to believe the drive gear isn't changed frequently.  I see in the chart above only two drivers are listed for the 400. Do you know the tooth counts? They do list a 34 tooth count driven,so I'm a 1/3rd of the way there. To e-bay in search of a adapter. btw my car is a 68, thanks for the support. Paul

Ed B

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Re: correct speedo gear
« Reply #8 on: July 10, 2006, 02:07:06 PM »
Yes, the 8623311 has 16 teeth and the 8623312 has 17, And by the way, the 68 Turbo 400 uses the same gears as the 69.

Ed