The relationship between gastric motility and the mixing of liquid food in the stomach is investigated using a numerical analysis. Three parameters of gastric motility are considered: the propagation velocity, frequency, and terminal acceleration of peristaltic contractions. We simulate gastric flow using an anatomically-realistic geometric model of the stomach, considering free-surface flow and moving boundaries. When a peristaltic contraction approaches the pylorus, retropulsive flow is generated in the antrum. Flow separation then occurs behind the contraction. The extent of flow separation depends on the Reynolds number (Re) which quantifies the inertial forces due to the peristaltic contractions relative to the viscous forces of the gastric contents; no separation is observed at low Re, while an increase in reattachment length is observed at high Re. While mixing efficiency is nearly constant for low Re, it increases with Re for high Re because of flow separation. Hence, the effect of the propagation velocity, frequency, or terminal acceleration of peristaltic contractions on mixing efficiency increases with the Reynolds number.