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Old 12-11-2004, 12:06 PM
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TH350 speedo gear color

Can anyone tell me what color gear (gears) I would need to have my speedometer to read correctly. Have a 77 vette that I just changed the differential gears to 3.55 and would like to change the speedo gears before I put eveything back under car. I had 3.08 gears/ 225/70-15 tires/27.4"dia.and drive gear on transmission is (Green 7T) and driven gear (Red 21 T). With the change to 3.55 gears what gear or gears do I need to change and to what? Thanks jw

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Old 12-11-2004, 02:31 PM
stu stu is offline
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http://www.tciauto.com/tech_info/speedo_gears.htm
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Old 12-11-2004, 08:55 PM
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I suspect it wasn't right to start with. Go to http://novaresource.org/speedo.htm , and plug your tire diameter & gear ratio figures into the formula, then pick a speedo gear combo that gives you the proper ratio.

I just did it & it looks like an 8-tooth drive gear with your 21-tooth driven gear would work.
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Old 12-12-2004, 12:03 AM
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i bought a trans a few years ago and they told me to yank and reuse my stock speedo gear to get a proper reading and they told me if i had changed rears you can go to a auto dealer and tell them what year+gear ratio and theyll give you the correct speedo gear to give you a correct speedo reading. like- i have a 72 truck with a 373 and theyll give you the corrrect speedo gear.
but if the overall diameter of your tires doesnt equal the original equipment it would be wrong.
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Old 12-12-2004, 01:22 AM
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Thanks I will contact my chevy dealer and see if they can order.Looks like I need the 8 & 21 gears
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Old 12-12-2004, 07:24 AM
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If you have any trans shops in your area, you might check with them. They usually have boxes of speedo gears laying around. They shouldn't charge you much, if anything at all.
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Old 12-12-2004, 06:33 PM
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Through the links and advice you are looking at acheiving a 2.60 gear reduction in the speedometer gears to correct your speedometer.
You can keep the 21 tooth (red) drive gear and switch the driven gear to an 8 tooth (orange) driven gear which will give you a 2.625 ratio.

That little 0.025 ratio difference equates to .7 mile per hour at 65 miles per hour, about as much difference as having brand new tires as compared to a set of tires at the minimum tread wear amount before needing replaced.

If you are in an area similar to mine you might be hard pressed to find the speedometer gear you need, no matter how common it may be. There are online sources for the gear you need, and a simple search will put you in contact with someone who has them.
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Old 12-12-2004, 07:57 PM
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Gear

Go to www.corvetteamerica.com, they have 11 different gears for your tranny 34 thru 45 teeth. All color coded.
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