Torque=Force X radius of the Gear
For example,
You apply 2400 ft-lb of torque
Force = 2400 ft-lb X 12"/5"= 5760 lb
You need 12000 lb-ft
The first Gear is 5" in radius
the second Gear you need 8"
The third Gear you need 15.625"
Or you can increase the torque from 19.2 ft-lb to 96 ft-lb