How is gear ratio calculated for a pair of gears, and what effect does it have on speed and torque?

How is gear ratio calculated for a pair of gears, and what effect does it have on speed and torque?

• A. Gear ratio = driven teeth / driving teeth; a ratio greater than 1 increases speed reduction and increases torque accordingly.
• B. Gear ratio = driving teeth / driven teeth; a ratio greater than 1 increases speed but reduces torque.
• C. Gear ratio = driven teeth × driving teeth; ratio affects only speed, not torque.
• D. Gear ratio = driving teeth minus driven teeth; ratio has no effect on speed or torque.

Answer: (A)

Gear ratio is the driven gear’s number of teeth divided by the driving gear’s number of teeth. If the driven gear has more teeth, the ratio is greater than one, which causes a speed reduction at the output and a corresponding increase in torque (ignoring losses). Conversely, a ratio less than one speeds the output up and reduces torque. For example, driving gear with 20 teeth and driven gear with 40 teeth gives a ratio of 2.0: output speed is halved, while output torque roughly doubles (minus losses). This is why the option using driven/driving teeth and stating that a ratio greater than 1 yields speed reduction and torque increase is the correct description. Other formulations—multiplying the teeth, subtracting them, or claiming no torque effect—do not reflect how gear trains actually transfer speed and torque.

Gear ratio is the driven gear’s number of teeth divided by the driving gear’s number of teeth. If the driven gear has more teeth, the ratio is greater than one, which causes a speed reduction at the output and a corresponding increase in torque (ignoring losses). Conversely, a ratio less than one speeds the output up and reduces torque. For example, driving gear with 20 teeth and driven gear with 40 teeth gives a ratio of 2.0: output speed is halved, while output torque roughly doubles (minus losses). This is why the option using driven/driving teeth and stating that a ratio greater than 1 yields speed reduction and torque increase is the correct description. Other formulations—multiplying the teeth, subtracting them, or claiming no torque effect—do not reflect how gear trains actually transfer speed and torque.