Industrial Mechanics Practice Test

In spur gears, the gear ratio is found by comparing the driven gear to the driving gear using their tooth counts. The standard convention is i = z2/z1, where z2 is the number of teeth on the driven gear and z1 on the driving gear. With z1 = 20 and z2 = 40, the ratio is i = 40/20 = 2. This means the driven gear turns at half the speed of the driving gear (ω2 = ω1 × z1/z2 = ω1/2). In an ideal system, the torque on the driven gear is increased by the same factor, so the power transmitted remains consistent.

So the gear ratio is 2. The other values would correspond to the reciprocal definition or different tooth relationships, but with these counts, 2 is the correct gear ratio.