My Exe 2-1/2" lathe has a 12tpi leadscrew and a 18 tooth gear permanently on the spindle. Simple gearing for a given TPI requires a leadscrew gear 3/2 times the desired thread
e.g. - required 16 tpi - 16x3/2 = 24t on leadscrew
The standard gears "rise by threes" - 18, 21, 24, 27, 30, 33 and...