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Discussion Starter · #1 ·
I recently swapped out my TH350 with an M20. The car the M20 came out of has 3.07 rear gears. I have 3.73. The M20 speedo gear looks to be an off white, almost with a yellow tint (but that might just be from the gear oil).

Can anyone help me determine what color speedo gear I need for the 3.73s?

Thanks, Jay

GR868
 

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Discussion Starter · #4 ·
Patrick, my speedo was never correct in the TH350 either.

But I think I figured it out. The number I was missing was the Drive gear #, which I calculated to be 8 from several online gear calculators. So 8(drive gear) x 3.73(axle ratio) x 758(rev/mile) / 1001 = 22.6 or 23 teeth.

Thanks

Jay
GR868
 

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What year M-20 do you have? Some of the older M-20 used a metal drive gear on the out put shaft, the 69 up ones used a plastice gear & a metal clip to hold the plastic drive gear in place. You will need to know for sure if you have a plastic gear or metal gear before you can do anything. Here is the Phone # of a man that helped me get my speedo right he is reasonable on his parts, you will need to know how many teeth on your drive gear, rear end ratio, tire diameter etc when you can Dennis his phone is 606 237 6648
 

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Discussion Starter · #6 ·
Johnnyr, the M-20 is out of a 69 Camaro It's kind of hard to see since it's under the car, but it looked like it was plastic and white in color.

Thanks for the info.

Jay
GR868
 

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There's a formula you use to calculate the speedo gear you need. First determine what gear is on the output shaft of your trans. Usually it's an 8 tooth. You also need to find out the diameter of the tire you're using.

I'll post the formula tonight since it's in my toolbox :) This will tell you how many teeth you need on your speedo gear
 

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When I went to a 308 yrs ago I messed up the calcuations, just couldnt get it right..so instread I brought a full set..they are not expensive...and not hard to change.
Tried a couple, compared by following a freind driving another car..and got close.
Then one day a Cop was doing his thing with the the radar gun...stopped had a chat with him...and he ran a check....
He did advise me to stay 5 miph under the speed limit when he checked, other wise he would have to 'ticket 'me if I was over..never knew if he was pulling my leg or not...I think he was thu.
 

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OK, so here's the fomula I promised

SGR= speedometer gear ratio

SGR= 63360 x axle ratio / 3142.6 x tire diameter

To determine the number of teeth you need on the driven gear, multiply the SGR x number of drive teeth on your mainshaft

example:
rear: 3.08
tire diameter: 26"

SGR = 63360 x 3.08 / 3141.6 x 26 = 2.389

For an 8 tooth drive gear:

8 x 2.389 = 19 teeth on the driven gear.

PS - the gearset used for the M20 and TH350 are the same. The plug, o-ring, and retainer on the transmission is the same also. Just determine what gear you have on the mainshaft and buy the appropriate driven gear. They usually cost about $5-10

hope this helps,
Mike
 

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Discussion Starter · #10 ·
Thanks for everyone's help. I purchased a 22 tooth gear yesterday from a chevy dealer and installed it late last night but have not tested it yet.

I wanted a 23 tooth, but would have need to change the drive gear too. This should put me very close.

Jay
GR868
 
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