 The velocity "v" of the point of contact on the pitch circles is the same on both gears, and is given by 
where input gear "A" has radius "rA" and meshes with output gear "B" of radius "rB,
therefore,
where "NA" is the number of teeth on the input gear and "NB" is the number of teeth on the output gear.The mechanical advantage of a pair of meshing gears for which the input gear has "NA" teeth and the output gear has "NB" teeth is given by

This shows that if the output gear "G"B has more teeth than the input gear "G"A, then the gear train "amplifies" the input torque.And, if the output gear has fewer teeth than the input gear, then the gear train "reduces" the input torque.