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Discussion Starter · #1 · (Edited)
My speedometer is way off and I have no idea how fast I'm going. I try to keep it on 80mph as that feels like 55mph. According to a website, since I'm running a 3.73 ratio and a 25.7" (245/40zr18) diameter tire that I need a .341 speedometer gear in my transmission. Also according to this website, that ratio is close to a stock ratio used in those transmissions. I guess my first step is to pull the on in my car now and check??
Here is the website:
http://www.montess.org/speedo_gears.html
I may need the 10/29 if mine has a 10/30.
Bowtie overdrives website says I need a 10/30
Any thoughts? Guess I will have to take the pan off to tell what gears are in there now?
Josh
 

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Trifive Automotive Electrical Wiring Expert
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That's the picture I was looking for. Their both in the tail shaft houseing. :smile:
 

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Discussion Starter · #5 ·
From my understanding, to remove the "driven" one, you basically remove the speedo cable, unscrew the bolt and pull the speedo drive assembly out and the "driven" gear is attached to it. The "drive" gear is on the actual spline shaft but I should be able to see it when I pull the "driven" gear out. Never done this before so I get to learn something.
Josh
 

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Don, on a 200-4R, you have to change the "drive" gear to get the right "range" for the "driven" gear in the side of the trans.

You have to drop the pan and the governor to change the "drive" gear.

I had to go to a 10 tooth "drive" gear on my Nova. It has a 3.31 rear gear. Don't remember the driven gear, probably different from this application. I think I use a 27 tooth black gear, but don't hold me to it. My Nova's tires are pretty short.

I used a chart like the one linked. Everything came out just right.
 

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Discussion Starter · #7 ·
Forgot to mention, i also have a VSS that attaches to the trans and my speedo cable attaches to the VSS. Don't think that would cause a problem but I should mention it. I'll be jacking the car up and pulling the "driven" gear this week to see what color and count it is. I should be able to see the "drive" gear as well.
Josh
 

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A 84 Monte SS with 3,73 gears a 200r4 had 215/65/15 that were 26 tall. Go to the Chevy store and buy the driven & drive speedo gears for this application, its not as hard as you are making it.

FYI my 56 has an accurate functioning speedo, however my 55 & other car I own do not even have speedo cables. Like NASCAR if I watch the tach I avoid speeding penalties
 

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Discussion Starter · #9 ·
Lol...2 years and still have not done this. I'm so slow. THanks for the info Rick and Don. I've been re-researching this so I can get it done in the next couple of weeks. Process is easy just picking the right gears can be a crapshoot.
Question, should the actual tire diameter be used or should the effective tire diameter be used in the gear selection calcs?
My actual is 25.717 and my effective is 25.254. I'll tell you why I am asking. If I use actual then I would need about a 29 tooth driven but if I use effective then it's a 30 tooth. One tooth is about 5% difference at 60mph so roughly 3-6 mph reading or something like that.
Josh
 

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Discussion Starter · #11 ·
Thanks,i assumed the effective/static load radius was the correct choice but not a big fan of assumptions. I ran my information through this calculator:
http://tri-5.chevyrides.com/speedo.php
I would need a 29 driven if I have a 10 tooth drive. I'll be dropping the pan to verify but I believe I have a green drive which is a 10 tooth.
Thanks
Josh
 
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