OK, Here's one that's got me stumped !!!!!!!!!
The existing speedo cable in my 68 is for an old powerglide.
I'm installing a Muncie M21, running 15" rear wheels, and a 3.73 rear gear in a big 10.
To make the speedometer work I need to know what type of speed gear to install in the muncie. I've been told everythinbg from 17T to 25T.
Local Chevy dealer parts men don't have a clue if it ain't in the computer.
And, will the existing speedometer cable work with the 4-speed.
Daised and comfused !!!!!!!!!!!!

Height of your tires on the 15" rims will affect the accuracy of the whole package, what is height of your tires?

I think they are right at 27"
The tires are p225 / 70R15 if this helps.
I'm not where I can go and measure the exact height.
Thanks

Hope this helps 3.55 has 23 teeth and 3.73 has 24 since you have 15"wheels the 23 tooth gear should put you close to being wright and the speedometer cable will work may have to reroute it because power-glides come out under brake booster muncies came out under wiper motor.Ricks camaro sells longer cables just for these swaps.They sell for about $32.00

try:
http://www.tciauto.com/tech_info/speedo_gears.htm

you should find what you need.

TTFN
Mat

on the cable, if your muncie has the speedo drive on the drivers side, your existing cable will screw right in and work fine. If it comes out of the trans on the passenger side, you'll need the longer cable.

If you're speedo was accurate with the powerglide, just pull that gear and use it. Keep your fingers crossed, if the gear inside the trans is a match, all will be well.