This paper will focus on the influence exerted by a nonuniform flow distribution at the inlet of oxidizers to catalytic converters on conversion efficiency evaluated channel by channel. To this aim the flow inside the whole domain, constituted by the exhaust manifold and an elliptic-cross-sectional pipe connecting it with the converter shell, is simulated by means of a three-dimensional fluid-dynamic viscous model. In this way, after assigning typical converter size and geometry (i.e., elliptic) the gas flow rate distribution can be described at its inlet surface, also varying the total mass flow rate. After calculating the flow field at converter inlet by means of a three-dimensional model, evaluation is possible of local flow distortion in comparison with the ideal conditions of constant velocity of the gas entering the honeycomb converter channels. The abovementioned distorted flow field is then assigned as a local boundary condition for another model, developed by the authors, able to describe, through a one-dimensional fluid-dynamic approach, the reacting flow into the converter channels. It was also shown that, due to this flow distortion, honeycomb converters are not uniformly exploited in terms of pollutants of different quantities to be converted in each channel (i.e., a nonuniform exploitation of all the metals coating the ceramic monolith). Finally, the positive effects determined by a diffuser upstream of the converter on flow distribution are analyzed.