m-21

[steve6755]

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]

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.

[jwbavalon]

Info is on the Chevelle site.  http://chevellestuff.net/tech/speedo.htm

[CantRepeat]

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.