Abstract: A gear reduction box designed to be integrated into an existing oil field pumping unit to reduce the gear ratio and thereby reduce the size of the motor needed to operate the pumping unit. More specifically, the invention is a gear reduction box including a base that can be mounted on an oil field pumping unit, a drive motor having a first sheave attached thereto, a pair of bearings mounted on the base and rotatably holding a drive shaft, a larger second sheave removably attached to one end of the drive shaft and connected to the first sheave by a first drive belt, and a third sheave removably attached to the opposite end of the drive shaft and connected to the oil field pumping unit by a second drive belt. The second and third sheaves can be removed and replaced with varying sizes of sheaves in order to adjust the number of strokes per minute of the oil field pumping unit.