To Calculate Torque

• 1). Multiply the engine torque by the original gear ratio to get effective torque. Example: if your engine has 300 foot-pounds of torque and your final drive ratio is 2.5:1, the effective torque to the axle is 750 ft-lbs.
  
  
• 2). Multiply engine torque by intended gear ratio. Example: you apply 3.5:1 gearing to that same 300 ft-lb. engine, your effective torque comes out to 1,050 ft-lbs.
  
  
• 3). Subtract new effective gear torque from old effective torque. In our example, an increase in gearing from 2.5:1 to 3.5:1 on an engine with 300 ft-lbs of torque yields an increase of 300 ft-lbs. to the wheels.
  
  

To Calculate Ideal Gearing

• 1). Determine the desired effective (wheel) torque. In this example, we'll say that you have the same 300 ft-lb engine and want 1,200 ft-lbs. of torque to the wheels.
  
  
• 2). Divide the desired effective torque (1,200) by the engine's output torque (300). In this example, that comes out to a gear ratio of 4:1.
  
  
• 3). Order a set of gears as close to that number as possible. For many domestic vehicles in an example like this, the closest thing to 4:1 comes out to 4.12:1.