A hypoid gear is a spiral bevel gear in which the axis does not intersect with the axis of the meshing gear, or pinion. The primary application for a hypoid gear is in the differential drive of a wheeled vehicle, where the drive shaft must be at a right angle to the wheels. The helical teeth in a hypoid gear produce less vibration than a gear with spur-cut or straight-cut teeth. Hypoid gears are manufactured in pairs and should be replaced in pairs.

A right-hand hypoid gear is one in which the outer half of the teeth are inclined in the clockwise direction as one looks at the face of the gear. Similarly, a left-hand hypoid gear has the outer half of its teeth inclined in the counterclockwise direction. A hypoid gear and its pinion usually have opposite handedness. This type of gear also can be classified according to its spiral angle — the angle between an element of the pitch cone and the tooth trace.

A hypoid gear has the shape of a revolved hyperboloid, meaning that its pitch surface forms a hyperbolic surface. Its pinion is off-axis with respect to the ring gear, also known as the crown wheel. This allows the pinion to be larger than the hypoid gear, which causes the pinion to have greater contact with the hypoid gear.

Hypoid gears are generally stronger and quieter than an ordinary spiral bevel gear. They can also handle a higher reduction ratio. The teeth in a hypoid gear experience some sliding, causing friction. This means that a hypoid gear requires special oils, which will lubricate the gear under very high pressure.

A larger offset in the hypoid gear increases its torque at the expense of efficiency. The most common use of hypoid gears is in older rear-drive vehicles, especially trucks. Designers of modern automobiles have tended to value the greater efficiency of an ordinary spiral bevel gear. The greater torque of a hypoid gear is most beneficial in larger trucks.

A spiral bevel gear must be much larger to develop the same torque as a hypoid gear. This generally means that a spiral bevel gear has less ground clearance than a hypoid gear, and it produces a greater hump in the floor of passenger cars. It's normally impractical to replace a hypoid gear with a more efficient spiral bevel gear.