In a three-gear drive with 60 teeth driving, an 8-tooth intermediate gear, and a 12-tooth driven gear, the overall gear ratio is

• A. 4
• B. 5
• C. 1
• D. 10

Answer: (B)

In a gear train, the overall speed change accumulates over each mesh, and for external gears the speed change is the inverse of the teeth ratio. So the intermediate gear doesn’t change the result net—its effects cancel when you multiply the two stages.

Here’s how it works step by step: the driver (60 teeth) drives the 8-tooth intermediate, which would make the intermediate spin faster by a factor of 60/8. Then the intermediate drives the 12-tooth driven, transferring that motion to the final gear with a factor of 8/12. Multiply the two stage factors: (60/8) × (8/12) = 60/12 = 5.

That means the driven shaft rotates five times for each rotation of the driving shaft. So the overall gear ratio is 5:1 in terms of speed (a speed-up of 5). In an ideal case, the torque on the driven gear would be reduced by the same factor (1/5) compared to the input torque.