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

You haven't found out it takes a 40 tooth gear until you drive it and it's accurate. Maybe it will be right. Maybe it will be a lot closer; and good enough.
Thank you. I gave Mancini the tire size/rear end gear ratio and that's what they came up with (40 tooth). Hopefully it will be close but we'll see. All I have to do now is figure out how to install. Heard it's fairly easy.