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The energy efficiency of external gear pumps (EGPs), similar to all positive displacement machines used for high-pressure applications, is significantly influenced by the power losses occurring in the lubricating interfaces that seal the internal displacement chambers.Therefore, it is crucial to account for these interfaces accurately, when developing predictive simulation tools.However, the literature has suggested various modelling approaches for EGPs, with different assumptions regarding the analysis of these interfaces.This makes it challenging for a designer or a researcher to determine what are the essential physical domains needed for properly modelling the lubricating interfaces and assess the EGP performance.This paper addresses the above research question by leveraging a comprehensive simulation tool (Multics-HYGESim) developed by the authors' research team to compare the effect of different modelling assumptions.HYGESim includes tribological considerations pertaining to the meshing of the gears, the lubricating films at the tooth tip interfaces, at the journal bearings, and at the lateral interfaces.It also considers realistic fluid properties, including the effects of cavitation and aeration, mixed lubrication effects, as well as material deformation effects for the gears, lateral bushings and the EGP housing.Deformation of the internal parts of an EGP is related to the internal balancing features and it is strongly coupled with the instantaneous pressurization of the pumping volumes.For this reason, a realistic quantification of these effects is difficult in simulation.Using a commercial EGP design as a reference, with known experimental volumetric and hydromechanical efficiency, this paper demonstrates how predictions can vary based on different simulation assumptions regarding body and lubricating film behaviours.Results are discussed starting from a basic rigid-body assumption that considers only body motion and analytical formulations of lubricating interfaces, to simulation model cases of progressively increasing in complexity to account for deformations of gears, bushings and housing.The results show that consideration of deformation effects allow more accurate prediction of power losses and efficiencies of the pump while simulations carried out without deformation considerations approximate the leakages and the power losses at the lateral lubricating interface though can predict the fluid dynamic performance.These findings will offer valuable insights to EGP designers, enabling them to understand the strengths and limitations of different modeling assumptions on the prediction of EGP behavior, especially regarding the effects of body deformation.