speedometer is off - Team Camaro Tech
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post #1 of 21 (permalink) Old Apr 6th, 02, 03:40 PM Thread Starter
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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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post #2 of 21 (permalink) Old Apr 6th, 02, 04:15 PM
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We have a local speedo shop that makes them up for us, any ratio you'd like...about $55.

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post #3 of 21 (permalink) Old Apr 6th, 02, 04:19 PM Thread Starter
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How do I figure the ratio. It is 15 mph fast, so what do I need to correct it. Thanks.
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post #4 of 21 (permalink) Old Apr 6th, 02, 04:39 PM
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Don
 
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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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post #5 of 21 (permalink) Old Apr 6th, 02, 04:44 PM
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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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post #6 of 21 (permalink) Old Apr 6th, 02, 05:42 PM Thread Starter
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Thanks for all the help! All replies are appreciated and I will search for past posts on this topic.
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post #7 of 21 (permalink) Old Apr 7th, 02, 08:18 AM
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either that or order the correct gears from Ricks first gen,
www.firstgen.com , download the transmission section of the catolog (pdf file) its on page 6 or 7, basically lists all the gears for year/trans/ and rear.

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post #8 of 21 (permalink) Old Apr 7th, 02, 11:35 AM Thread Starter
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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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post #9 of 21 (permalink) Old Apr 7th, 02, 03:14 PM
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Hey Yogi...Try out this Speedo-Calculator. Hope it helps...
http://www.700r4.com/SpeedOCalc/

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post #10 of 21 (permalink) Old Apr 7th, 02, 05:38 PM
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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

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'65 327/275 Corvette Coupe(previously)
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post #11 of 21 (permalink) Old Apr 7th, 02, 07:02 PM
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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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post #12 of 21 (permalink) Old Apr 8th, 02, 08:07 AM
 
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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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post #13 of 21 (permalink) Old Apr 9th, 02, 03:24 PM Thread Starter
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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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post #14 of 21 (permalink) Old Apr 10th, 02, 02:39 AM
 
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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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post #15 of 21 (permalink) Old Apr 10th, 02, 07:45 PM
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http://www.geocities.com/MotorCity/S...432/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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[This message has been edited by davidpozzi (edited 04-11-2002).]
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