Author Topic: m-21  (Read 397 times)

steve6755

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m-21
« on: April 16, 2022, 04:24:39 PM »
can anyone provide info on the 1969 Z28 m-21 speedo "drive gear" size (seems there are 2), just changed the rear end gear from 4:10 to 3:42 and need to change the driven gear to match the new rear gear

firstgenaddict

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Re: m-21
« Reply #1 on: April 16, 2022, 06:07:02 PM »
Not only are there more than one... there are adapters which change the speed as well. There was never a 3.42 available with the 69Z you are going to have to figure out your current speed using someone pacing you then convert to how much slower/faster you need the driven gear to be spinning. 

One thing to note - If your tire height is not the same as OE (approx 26") then you are already off.
James
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jwbavalon

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Re: m-21
« Reply #2 on: April 16, 2022, 07:04:42 PM »
Info is on the Chevelle site.  http://chevellestuff.net/tech/speedo.htm

CantRepeat

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Re: m-21
« Reply #3 on: April 17, 2022, 10:56:00 AM »
Not only are there more than one... there are adapters which change the speed as well. There was never a 3.42 available with the 69Z you are going to have to figure out your current speed using someone pacing you then convert to how much slower/faster you need the driven gear to be spinning. 

One thing to note - If your tire height is not the same as OE (approx 26") then you are already off.

Could just install a GPS speed app on your phone to get accurate speed and use that for your calculations.
-Tim
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