In a spur gear set with a 40-tooth driver and a 60-tooth driven gear, what is the gear ratio and the output speed relative to input?

• A. Gear ratio is 2:3; output speed is 2/3 of input
• B. Gear ratio is 3:2; output speed is 3/2 of input
• C. Gear ratio is 1:1; output speed equals input
• D. Gear ratio is 4:1; output speed is quarter input

Answer: (A)

In a spur gear pair, the output speed is inversely related to the number of teeth on the gears, and the gear ratio is taken as the driver-to-driven teeth. With a 40-tooth driver and a 60-tooth driven gear, the ratio simplifies to 40:60 = 2:3. This means the driven gear turns more slowly. The output speed is the input speed multiplied by 40/60, which is 2/3. So the output runs at two-thirds of the input speed, and the gear ratio is 2:3. (If you instead describe the ratio as driven-to-driver, you’d see 3:2, but the stated convention here uses driver:driven.)