Abstract: A gear reducer, especially for a mixer, has a housing supporting a pinion shaft with a pinion gear rotatably driven by a motor, mechanically coupled with a rotatable driven shaft having a driven gear meshing with the pinion gear. The driven gear can be coupled, for example, to the impeller of a mixer via a vertical impeller shaft. The pinion shaft is carried in a cartridge that is adjustably mounted to the housing for moving the pinion shaft toward and away from the driven gear and for accommodating pinion gears of different diameters to change the gear reduction ratio without the need to change the driven gear on the impeller shaft. The cartridge is pivotally mounted to the housing at a point spaced from the axis of the driven shaft such that the cartridge and the pinion gear can be rotated toward and away from the driven gear. This also permits intermediate reduction gears that may extend below the driven gear to be moved clear of the driven gear such that the cartridge can be lifted free.