How do you know what speedometer cable transmission gear to use?

You need more teeth on the gear. If you have an extra one, stick it in there and see how much it changes. To get this right, you have to know exactly how far off it is now and how many teeth are on your present gear. And read the numbers on the speedo gear housing that is in the trans. You'll see there are numbers, and nubs to line up depending on how many teeth the gear has. Write down how far off it is now, what the teeth count is, what the teeth count of any gear you install, how far off that one is and on...The charts will get you close, but sometimes there isn't a gear available that will make it perfect. And sometimes they're expensive; depending where you shop.