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Speed ratio as the inverse of gear ratio

Aviation Mechanic General (AMG) practice question 2 of 25 on Mathematics. ACS element AM.I.H.K4 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONTwo gears in an aircraft component have a gear ratio of 2:9. What is their speed ratio?

  1. A2:9
  2. B9:2
  3. C1:4.5
Show the answer and explanation

Correct answer: B. 9:2

  • A — incorrect 2:9 is the gear ratio itself; the speed ratio is its inverse.
  • B — correct The speed ratio of two gears is the inverse of the gear ratio, so a gear ratio of 2:9 gives a speed ratio of 9:2.
  • C — incorrect 1:4.5 is just the gear ratio 2:9 rewritten with a numerator of 1; it has not been inverted.

BACKGROUND

Gear ratio compares the number of teeth on two meshing gears. As the number of teeth in a gear decreases, its rotational speed increases, and vice versa. Therefore the speed ratio of two gears is the inverse (or opposite) of the gear ratio. The handbook's example states that gears with a gear ratio of 2:9 have a speed ratio of 9:2. This inverse relationship is used to calculate the rpm of a driven gear from the rpm of the driving gear.

STUDY TIP

When a question moves from teeth to rpm, flip the ratio; fewer teeth means faster rotation.

KEY POINTS

  • Gear ratio compares teeth; speed ratio compares rpm.
  • Speed ratio is the inverse of the gear ratio.
  • Gear ratio 2:9 gives speed ratio 9:2.

SOURCES

  • FAA-H-8083-30B ch03 "Aviation Applications" (Ratio: gear ratio and speed ratio)
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.