I know started a similiar thread yesterday but I didn't have all the info I needed. I have an M-21 with a speedo drive gear of 1.76 O.D. and 8 teeth. The driven gear is kinda green with 20 teeth and also has a ratio adapter with a 1.824 ratio. Transmission mechanic says he thinks the drive gear is too small in diameter because the teeth just barely engage and click when a little pressure is holding the cable back. I think that the driven gear is wrong. Someone please explain what I need to make this work with the 8 tooth drive and 1.824 adapter. What driven gear do I need??? Thanks in advance, Kevin The rear is a 4.56 and Im planning on 14" stock size tires.

You probably have a driven gear for a 1.84" drive gear. Go here for help: http://chevelleengineer.home.comcast.net/speedo.htm

also try your gm dealer sometimes the parts department will look it up if they not lazy azzes give them gear ratio tire size and they will look it up

This site had all the information I needed to fix the same problem:

http://www.tciauto.com/Products/TechInfo/speedo_gears.asp


They also have a couple of easy to use calculators to help you determine what speedo gears you need.

thanks for the help guys. If I had not seen the ratio on my adapter, I would not have been able to calculate it. I have not found that particular adapter on a chart yet. Kevin