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Discussion Starter · #1 ·
I just finished my 69 and my speedometer is 15 mph fast. I have a 350 turbo with a 373 posi rear, tires are 235 60 x 15. I was told that you can buy a gear reduction adapter that hooks to the cable. I cant seem to find any info on this. Has anyone used one and where can I get one. Thanks.
 

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Discussion Starter · #3 ·
How do I figure the ratio. It is 15 mph fast, so what do I need to correct it. Thanks.
 

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You could put it the rollers that a few of the speedo shops have now, or a ballpark method is to use the mile markers on a highway, using your odometer readings compared to "actual" miles traveled.
Just as an example, let's say you traveled 100 miles on the highway, and your odometer says you've traveled 115...you'd need an adapter to correct a 15% error.
Of course, this is assuming your speedo head and odometer "jive".

I'm sure there are a lot of different methods, but this works.

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Don~ ZZ430DropTophttp://hometown.aol.com/zz430droptop67rs
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67 RS/SS Convertible, 70 RS/Z28
 

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Yogi,

Just pull out your speedometer cable from your tranny and replace the speedometer gear with one that has less teeth. Generally you'll get 3 mph reduction per tooth.

This topic has been covered before. Search the site to get technical details.

Dave
 

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Discussion Starter · #6 ·
Thanks for all the help! All replies are appreciated and I will search for past posts on this topic.
 

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Discussion Starter · #8 ·
I am looking at the catalog right now. They have a driving gear and a speedometer gear. The speedo gear lists one for 3:73 rear but how do I know which driving gear to use? They list 4 different sizes. Thanks.
 

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Hi yogi.

I wouldn't worry about the output gear unless you plan on tearing down your transmission. That gear is located on the output shaft of the trans and can only be replaced by pulling the tranny apart. You should be pretty close with a 24 tooth gear. I just finished rebulding a M-20 and used a 24 tooth gear for my 69 with the same 3:73 rear and well/tire combo as you have. I haven't installed my new tranny yet but all my calculations came up with that result.

Good luck.

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Doug

'69 SS clone 350/300
'65 327/275 Corvette Coupe(previously)
 

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on a TH350 it is easy to get to the speedo gears- drop the driveshaft, take off the tailshaft, and you are there. the gear is held on with a metal clip that you can remove with you fingers or a small screwdriver.
all you need to do is figure out what your engine speed is at 60 MPH, then divide by 1000. this is your final drive ratio. if it is 2000 at 60, for example, you have a 2:1 drive ratio. with that, if you had a 10 tooth drive gear, you would need a 20 tooth driven gear. a little easy math is all you need to do. just count the teeth on the drive gear, and divide by your drive ratio, and get a driven gear with that many teeth on it.
sorry, but i can't find my Auto Math book to tell you the formulas, but i have posted it all here before. do a search, and you should be able to find it.

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1971 Nova(looks like 69 camaro from underneath!)
355sb, vortec heads, HOT cam,T-10 tranny, 3.70 gears 16X8" IROC wheels. 12" Corvette brakes on the way.
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There is a place called, Bob's Speedometer Service, ph#248-473-9006. It is in Farmington Hills, Michigan. If you call, they will ask you a few questions about your rear end gear ratio, tire diameter, etc. Then for about $35 they will build you a speedo reduction cluster, it is in a small aluminum box that hooks up in line between your speedo cable and trans. If you provide the correct info, they will gaurentee speedo accuracy. I was happy with their service.

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camarodave
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Discussion Starter · #13 ·
Thanks to everyone. This is a great place to learn all kinds of stuff. I think I will go with the reduction cluster idea. It seems to be the easiest way to do it. Thanks for all the information.
 

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Ok maybe I look at this different. I have replaced my trans in my 67. To recal the speedo I did two things. One had a pace car dive with me doing 45 then 65 and note the speed difference from mine to theres. Then take out speedo cluster. Put on bench and use an old speedo cable attached to a drill. Run the drill up to max and hold. Once it stablizes, adjust the speedo + or - the amount that you calulated from the pace car. The adjustment is done with the moveable tab on the back of the speedo. Once in the car you can check it by useing a stop whatch and the mile markers on the road. 60 MPH = 1 minute per mile. It may not be the best way but it will get the job done.
 

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http://www.geocities.com/MotorCity/Speedway/6432/speedo.htm
Check the Chevelles.com site for a similar chart and method. I think the th 350 is the same as a four speed in the spedo gears. Just get a driven gear that is five teeth more than what you have in there now. About three MPH change for each tooth on the driven gear.
David
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Check my web page for First Gen Camaro suspension info:
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67 RS 327 original owner. 69 Camaro Vintage Racer, 65 Lola T-70 Chev SB Can-Am Vintage Racer



[This message has been edited by davidpozzi (edited 04-11-2002).]
 

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Yogi,

I agree that more teeth are needed. Please consider this, if the gear only had a hand full of teeth it would complete one revolution much faster than if it had say 100 teeth (using the same drive gear). Yogi needs to slow the speedo cable down so more teeth are required than what he currently has. I still think 4 or 5 teeth more should be real close based on your wheel and tire size. I also do not believe you need to do anything to your drive gear.

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Doug

'69 SS clone 350/300
'65 327/275 Corvette Coupe(previously)

[This message has been edited by GDV350SS (edited 04-11-2002).]
 

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Discussion Starter · #19 ·
I put in a 24 tooth gear and it is only 5 mph fast now. It had a 21 tooth in it. I guess the tire size is probably why it is still off. I can live with 5 mph. Thanks for all the help.
 

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Yogi, your speedo may work for sometime but it will stop working or start to act funny. The problem is that you cannot use a large driven gear on a large drive gear for long. You had a 21T small driven gear that mates to a large drive gear, now you have put in a large driven gear(24T) but did not swap out the drive gear for a smaller one. The picture shows a large 25T and 24T and a small 21T. The small drive gear(blue)is for the large driven gears and the large drive gear(Lt green)is for the smaller drive gears.

http://www.villagephotos.com/pubimage.asp?id_=216533




[This message has been edited by secndgn (edited 04-15-2002).]
 
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