The larger gear in a pair is called the gear, the smaller is called the pinion.
Gear ratio: The number of teeth in the gear divided by the number of teeth in the pinion. Also, ratio of the speed of the pinion to the speed of the gear. In reduction gears, the ratio of input to output speeds.
Gear efficiency:Ratio of output power to input power. This includes consideration of power losses in the gears and bearings and from windage and churning of lubricant.
Speed: In a given gear, normally limited to a specific pitchline velocity. Speed capabilities can be increased by improving accuracy of the gear teeth and by improving balance of the rotating parts.
Power: Load and speed capacity is determined by gear dimensions and by type of gear. Helical and helical-type gears have the greatest capacity (to approximately 30,000 hp). Spiral bevel gears are normally limited to 5,000 hp, and worm gears are usually limited to about 750 hp.