Abstract: A rotary displacement pump having a rotatable gear rotor is disclosed, the gear rotor having an outer ring gear with a plurality of internal teeth engaging the external teeth on a pinion gear which is, in turn, attached to a rotatable shaft. The ring gear has an outer diameter that is approximately equal to the diameter of the root circle of the internal gear teeth such that the plurality of teeth are circumferentially spaced apart to form generally radial passageways. The ring gear has one or more annular portions connecting the plurality of internal gear teeth. Each of the internal gear teeth has a forward portion and a rear portion, measured in the direction of rotation of the ring gear, with a tooth profile having a convex forward portion with a radius R1 and a convex rear portion having a radius R2 such that R1>R2.