Large or small tailshaft?

[Clsccmro]

I'm trying to figure out if I have a large or small tailshaft on my th350 auto tranny. And even more, what speedo gear I need to purchase to correct my speedometer reading incorrectly. How do I find this information? According to a chart I used from the chevelle forum I need a 20 tooth speedo gear, but that's only if I have a 7 tooth gear mated up to it inside the tranny apparently, so how do i find out if that's what's in my tranny without ripping it apart? Any info is greatly appreciated. Thanks a lot!

[Clsccmro]

Anybody have any thoughts on this?

[HILROD]

The short tail is around 6 or so inches long, the medium is around a foot, then there is the long tail. You should be able to look in the speedo hole and see the color of the inner gear, it will let you know the teeth count. Some times you can't get it close enough with only an outer gear change. If this is true, you would have to remove the tail housing, and change the inner gear. I've been told you can't change all types of inner gears without dis-assembling the trans, but I've never had to do that. Easier is to get it closer with just an outer gear and get a ratio adapter. It plugs in, inline with the cable to speed up or slow the signal down. Even GM used these from the factory on some cars.

[Clsccmro]

Once again Hilrod, you're the man!

thanks for the helpful info!

[Skip Fix]

Roughly a 1 tooth per 5 mph @ 55mph change will get you close. There are two different gear housings depending on the gear tooth count. I forgot where the break is. Then there are different drive gears on the output shaft that are a little harder to swap out.