Accurately measuring the viscosity of a solids suspension requires uniform suspension of the solids in the viscometer cup. In a cup-and-impeller viscometer system, solids may settle when the impeller speed is too low, causing viscosity measurements to appear lower than that of a well-suspended slurry. Pre-mixing of a solids suspension is typically performed to achieve steady state prior to measurements. Data here shows that the measured viscosity values differ depending on the pre-mixing speed, indicating that the solids are not properly suspended at all speeds. A commonly used cup-and-vane impeller system can be thought of as a mixing tank that should operate above the uniform-suspension speed (USS), although determining the USS experimentally is rather subjective. Computational fluid dynamics (CFD) is employed here to determine the USS of a pretreated corn stover (PCS) solids suspension and to confirm the experimentally measured USS.