I need to change the speedometer gear on my 67. Its off by 15mph. What is the procedure for doing this?
The WCCC website has a calculator for determining the best driven gear tooth count.
They also sell the full range of gears as well as the retainers. These mount on the transmission end of the speedometer cable and are relatively easy to change.
I would remove the cable from the transmission. Count the teeth of the existing gear. Which way is it off slow or fast? 15 MPH is a lot.
It is reading 15mph faster. Its not the stock rear end or tires. I did use the calculator and it says I need 23 teeth. I wanted to know what is involved in changing it?
You remove one bolt, pull the speedo cable out of the transmission, remove the clip, slide the gear off, slide the new one on and reverse the process.
It’s pretty simple. After you remove the one bolt you can remove the metal clip at the end of the plastic gear. The gear then pulls off and you can put the new one on and reinstall the clip and the bolt.
Not sure if they make one with 23 teeth.
Sounds easy enough, thank you.
I have a '67 with the stock (3.00?) open rear end. 1 tooth was good for somewhere between 3 and 4 mph as I recall. Since I’m an engineer I just ran the calculations (revolutions, tire diameter, speed, time etc) 
I do not know how many teeth you can go up or down without changing the drive gear
I took our 67 to a speedo shop and they built a gearbox that goes between the speedo cable and trans. They can get within 1/2 % or better