Final answer:
Ring and pinion gears are sold together to ensure they match in terms of gear pattern and operate efficiently as they are machined to precise tolerances and work in tandem within a vehicle's differential.
Step-by-step explanation:
Ring and pinion gears are always sold together because these components work together in a vehicle's differential, where they must mesh perfectly to provide the correct gear ratio and smooth power transmission. The ring gear is attached to the differential case and turns with the wheels of the car, while the pinion gear is connected to the driveshaft and provides the torque from the engine. They are machined to very precise tolerances to ensure that they operate silently and efficiently. Over time, the gears wear down due to metal-to-metal contact and must be replaced as a set to match the exact gear pattern and prevent issues such as noise, inefficiency, or failure.