OldGMJon:
I am in the middle of this problem as well. A couple of thoughts:
* There are two sizes to these speedo gears. The ones readily available are .78" in diameter. My tranny uses a large gear that is .87" in diameter. You will need to pull out your gear and measure it to see what the diameter is. Then you will need to count the number of teeth on the one you have. The general rule is to get a 5% change in the speedo, you need to change this gear by one tooth. One tooth smaller makes it go faster on the speedo. So 42mph/50 mph = 84%. That would mean that you want a gear 3 teeth smaller than what you have to speed up your speedo 15%. Apparently, the factory generally checked the speedo's at 60 mph, but your 50mph check should be close.
* If you have the .87" gear, you will have problems finding a gear smaller than 22 teeth. Apparently, in that size they are no longer available. They do come up occasionally on E bay, but they are used gears and command a premium price if you can find them. The speedo shops recommend that you can pull off your tailshaft housing and replace the gear on the tranny tailshaft so that you can use the smaller "driven" gear that is shown in the pictures. Otherwise the external ratio adapter that one of the other folks referenced is a solution as well.
I hope this helps. This website was the best that I could find to sort this issue out:
http://chevelleengineer.home.comcast...eer/speedo.htm
I hope this helps.
Regards,
Steve
New Orleans