What is the approximate reduction ratio between armature shaft and output speed of a 39MT starter motor?

3.5:1

Explanation:

Of course, a 4:1 gear reduction ratio also means that a gear reduction starter can often produce more torque than a much larger, heavier direct drive starter. In some cases, a direct drive starter can weigh as much as two times more than a comparable gear reduction unit. That represents a significant power/torque to weight ratio benefit, but it also means they are physically smaller and often easier to install.