The differential is a system of gears that allows different drive wheels (the wheels to which power is delivered from the engine) on the same axle to rotate at different speeds, such as when the car is turning. A differential is also the component through which a rotating driveshaft can change direction; for example, in a rear-wheel-drive car whose driveshaft runs the length of the car, it joins and transfers its power to an axle that runs perpendicular to the car to drive the left and right wheels. Differentials are low-maintenance, sealed components that are expected to last the life of the vehicle but may have a recommended service interval to check and/or replace the oil. Note that a car with four-wheel drive can have two or more differentials. A failing differential can cause vibration, binding during turns and/or sounds ranging from clunking to howling.