If you pull the sender you can look in trans to see what gear color is as you will have two gears that makeup the ratio.
A drive gear (one in trans on shaft) and driven gear (one on sender your speedo cable is connected to).
Check out the colors it tells you what the ratio setup is.
Here is a example from the 700R4 trans to get you started.
Until we know the rear end gear ratio and transmission type / sender type. We can advise on what ratio you need.
Here is a site to input all those values and it will let you know what you need
https://transmissioncenter.net/shop/speedometer-gear-calculator/
