Speedometer Gear Problem

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  #1  
Old 09-13-02, 03:23 PM
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Speedometer Gear Problem

1979 Olds Cutlass Supreme Brougham, 305 Chevy Engine, 2:41 rear end, 20575-R14 tires.

I previously posted about changing a 200 Metric to a 3506C tranny. I was told the only problem would be the addition of a vacuum line. Tranny is installed but the speedometer gears are wrong.

The original gears had 7 and 19 teeth. This setup didn't work so the mechanic tried several other options that also didn't work. He currently has an 8 and 23 tooth gear installed. It is reading too slow.

Does anyone know what set of gears I need to make the speedometer accurate?

Thanks,

Sherm
 
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  #2  
Old 09-13-02, 08:13 PM
Joe_F
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First, wrong size tires. I believe stock is 19575R14. Changing the tire size has an effect.

Do a search on "speedometer calibration" on google.com. There is a formula to figure it out. You are on the right track. It's just a matter of finding the right gear part # from GM and installing it .
 
  #3  
Old 09-13-02, 08:45 PM
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Speedometer Gears

I drove the car tonight and there was a clicking noise. I think it's from the speedometer cable or gears. I remember this from a car years ago and lubricating the cable stopped it. The noise also could be from the wrong gears. I noticed the needle is jumpy instead of smooth.

Any ideas to help?


Sherm
 
  #4  
Old 09-13-02, 08:53 PM
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In regards to the tire size, I listed it incorrectly. They are 205/70/14 not 205/75/14. The lower profile makes them closer to the specs. Is this enough to matter?

Assuming the tire diameter is less than spec they should turn faster and make the speedometer fast. Is this correct? Does 205/70/14 have a smaller diameter than 195/70/14?
 
  #5  
Old 09-13-02, 09:15 PM
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Tire Diameter Calculation

Here's the calc for tire diameter with 205/70/14 tires:

205 x .70 =143.5 x 2 =287 cm
287 cm/25.4 =11.299" + 14 = 25.299 or 25.3

Based on 1000 revolutions/minute I need a driven gear with 29 teeth and a drive gear with 15 teeth. I haven't checked the revolutions/minute yet, it's dark now. I'll do that tomorrow and adjust as necessary.

Do you agree with these calculations?

Thanks,

Sherm
 
  #6  
Old 09-14-02, 06:20 AM
Joe_F
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Yes, that sounds right. Any deviation from the stock spec will effect speedo calibration.

Now, simply go to any GM parts counter with that information and find a driven and drive gear with that # of teeth. There's a chart in the GM parts catalog that lists color, # of teeth, and part #.

They should all still be available. Pitch that speedo cable for a new one, sounds shot.
 
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