In a gear drive with a 12-tooth driver gear and an 18-tooth driven gear, the gear ratio is which value?

• A. 0.8
• B. 1.5
• C. 2.0
• D. 3.0

Answer: (B)

The gear ratio here reflects how the speeds of the two gears relate to their teeth counts. With a 12-tooth driver and an 18-tooth driven gear, the input-to-output speed ratio is determined by the number of teeth on the driven gear divided by the number of teeth on the driver gear. That gives 18/12 = 1.5.

So for each turn of the driver, the output turns 12/18 of a turn (two-thirds of a turn). In other words, the driver must rotate 1.5 times to make the driven gear complete one revolution. In an ideal gear pair, the torque shifts by the same factor, so the output torque increases about 1.5 times compared to the input torque.

Choices that would require the driven gear to have fewer teeth than the driver (or imply a different arrangement) don’t match this 12-to-18 setup, which is why they’re not correct for this pair.