Every hydraulic system use requires a pump to supply the necessary flow and support the imposed pressure cycle. Therefore the performance of the pump in a hydraulic system is critical to the overall performance of the system. It is a well known fact that the performance of a hydraulic pump is influenced by both the speed at which it is operating and the pressure to which it is subjected. This paper evaluates the volumetric efficiency of a hydraulic pump using both the fundamental expressions which are generally accepted and actual data test data. The widely accepted equation for pump leakage is presented along with an expression that includes the parameter of speed. Results obtained using a pressure-loaded gear pump are used to compare and evaluate the analytical expressions. This comparison reveals a significant influence of speed at high pressures on the volumetric efficiency of the pump. Many times in actual practice the effect of speed on pump performance is not given proper consideration. These data indicate that for realistic performance analysis, the rotational speed of the pump is an important parameter.