The gear ratio is simply a measure of how many number of turns the output shaft makes when the input shaft turns once.
For our Sportster transmissions, the Gear Ratio is the ratio between the number of teeth on the two gears that are meshed together in the gearbox.

• It's important to distinguish between the input and output sources as these two conditions change during a ride.
  • The input source to the transmission with the clutch engaged (clutch lever released) is the engine.
    When you are on the throttle, the motor is driving the transmission and thus the rear tire.
    The motor is driving the mainshaft and it's gears, and the mainshaft gears are driving respective countershaft gears and thus the rear tire.
  • The input source to the transmission with the clutch disengaged (clutch lever pulled) is the rear tire.
    When you disengage the clutch, the motor is now not attached to the transmission but the transmission gears still turn.
    The rear tire is now driving the countershaft and it's gears which turn their respective mainshaft gears.
• When you let off the throttle with the clutch engaged (lever released), the rear tire is still trying to drive the transmission at the speed the motor was before pulling the clutch.
  So there is a push/pull going on inside the transmission while riding depending on throttle load.

• Final Drive Ratio:
  The final drive ratio is the last bit of gearing between the transmission and the rear wheel.
• Gear Ratios: Gear ratios represent the gears’ relation to each other in size. ¹⁾
  When different sized gears mesh together, they can spin at different speeds and deliver different amounts of power.
  When the input gear rotates, it transfers its rotational energy to the output gear. The gear ratio determines the speed and torque of the output gear.
  Gear ratio is normally expressed as an ordered pair of numbers separated by a colon, for example, 3:1 (or 3 to 1).
  A Gear Ratio can increase the output torque or output speed of a mechanism, but not both. ²⁾
  • Ratio between 2 gears:
    The ratio of a 2 gear train is the rate of the number of teeth on the driven gear to the number of teeth on the driving gear for our purposes. ³⁾
  • Internal Gear Ratio:
    Number of mainshaft revolutions required to drive the output sprocket 1 revolution. ⁴⁾ This number does not include the rear wheel.
  • Overall Gear Ratio:
    Number of engine revolutions required to drive the rear wheel 1 revolution. ⁵⁾
• Drive and Driven Gears: Determining which is which is crucial in calculating the gear ratio.
  Example 1: A 20T gear drives a 40T gear at a ratio of 0.5:1 - driven gear turns 0.5 (or 1/2) revolution for every 1 full revolution of the driving gear.
  Example 2: A 40T gear drives a 20T gear at a ratio of 2:1 - driven gear turns 2 full revolutions for every 1 full revolution of the driving gear.
  • Driving Gear:
    (aka input gear), The driving gear is the gear that transmits power.
  • Driven Gear:
    (aka output gear), The driven gear is the gear that receives power.
• Gear Train :
  A Gear train consists of two or more gears in series. It's application is to increase or decrease the speed or torque of the output shaft. ⁶⁾
• Input Shaft:
  The input shaft is the shaft that is transmitting power (the mainshaft when clutch is engaged or the countershaft when clutch is disengaged).
  In 5th gear, the power is transferred entirely through the mainshaft at a 1:1 ratio. So in 5th gear, the mainshaft is the output shaft.
• Mechanical Advantage:
  A mechanical advantage refers to an increase (or decrease) in torque or force that a mechanism achieves through a power transmission element. Gear ratio is used to define the mechanical advantage. Mechanical advantage is a measure of the ratio of output force to input force in the gear train in this application. ⁷⁾ Despite changing the forces that are applied, the conservation of energy is still true and the output energy is still equal to the input energy.
• Torque Ratio:
  The torque ratio is the rate of the torque of the driving gear to the torque of the driven gear. ⁸⁾ It determines the torque of the system.
• Speed Ratio:
  The speed ratio is the ratio of the angular velocity of the driving gear to the angular velocity of the driven gear. ⁹⁾ It determines the speed of the system.
• Speed (RPM):
  The rotational speed of the shaft generated by the motor, expressed in rpm (rotations per minute). ¹⁰⁾ The shaft speed in a gearmotor is proportional to the gear ratio. The output speed can be found by dividing the input speed by the gear ratio. Gear ratios above one will reduce speed, while those below one will increase speed. Another explanation: In a simple gear system, two gears with different numbers of teeth are meshed together. ¹¹⁾ When the input gear rotates, it transfers its rotational energy to the output gear. The gear ratio determines the speed and torque of the output gear. A gear with fewer teeth than the input gear will rotate faster and provide less torque, while a gear with more teeth will rotate slower and provide more torque.
• Spur Gears:
  The spur gears used in Sportsters have straight cut teeth positioned with mated pairs on parallel axis.
• Output Shaft: The output shaft is the shaft that is receiving power (countershaft when clutch is engaged in 1st-4th gears) or (mainshaft in 5th gear).

• Gear Ratio = (number of teeth on driving gear) / (number of teeth on driven gear) - result tells how much faster or slower the driven gear turns.
• Output Speed = (input speed) x (gear ratio)
• Mechanical Advantage = (number of teeth on driven gear / number of teeth on driving gear)
• Output Torque = (input torque) x (mechanical advantage).

The information below shows how to calculate the numbers for a gear train using only 2 gears.
Instead, you can find a gear ratio calculator online to do the math for you.
Here is a Link to one that gives you gear ratio, mechanical advantage, output speed and torque by inputting known values.

The examples below are using a MS-2 (29T) and a CS-2 (34T) gearset (or gear train). Engine RPM at constant 1000 and initial torque of 50 ft/lbs.
In this gearset, M2 is splined to the mainshaft, turns the same speed as the mainshaft and serves as the driving gear of the set.
C2 is sleeved over bearings on the countershaft, has to be coupled to the countershaft by the dogs on C3 gear and serves as the driven gear to M2.
These are arbitrary combinations just for example.
¹²⁾

• Gear Ratio = (number of teeth on driving gear) / (number of teeth on driven gear)
  • Example: A 29 teeth gear is driving a 34 teeth gear at a ratio of 0.85:1
    Plug the numbers into the formula (29 / 34) = 0.85294 (or 0.853)
    The result is expressed in ratio as 0.853:1 (or 0.853 to 1).
    In this example, CS-2 turns (slower) and at 0.853 times the speed of MS-2.
    While this setup shows a gear reduction in terms of speed, it creates an output that has more torque when compared to the input torque between these 2 gears.
• Output Speed = (input speed) x (gear ratio)
  • Example: Given a mainshaft speed of 1000 rpm using the gear ratio above (0.853), the countershaft speed is 853rpm
    Plug the numbers in the formula (1000 x 0.853) = 853
    In this example and with a constant input rpm, the countershaft (by way of CS-2) has a speed of 853 rpm.
• Mechanical Advantage = (number of teeth on driven gear / number of teeth on driving gear)
  • Example: A 34 teeth gear is being driven by a 29 teeth gear at a mechanical advantage of 1.172:1
    Plug the numbers into the formula (34 / 29) = 1.172413793103448
    In this example, the result is expressed in ratio as 1.172:1 (or 1.172 to 1).
    This example gives 0.172 times more output torque as compared to the input torque between these 2 gears.
• Output Torque = (input torque) x (mechanical advantage).
  • Example: Given an input torque 50 ft/lbs, the output torque will increase to 58.6 ft/lbs between the example gears
    Plug the numbers into the equation (50 x 1.172) = 58.6
    In this example, the torque from the mainshaft was increased on the countershaft by 8.6 ft/lbs (by way of CS-2).

AKA, gearbox ratio, the internal ratio includes the mainshaft and countershaft gears that are in the power flow for the transmission GEAR selected.

To find out what 2nd gear from above did to the final drive sprocket speed, this example uses 4 gears, since the power flow for 2nd gear uses all 4 of these.
This example uses the MS-2 (29T) and a CS-2 (34T) gearset from the above example plus CS-5 (25T) and MS-5 (42T).
And still using engine RPM of a constant 1000 and initial torque of 50 ft/lbs.