The magnetorheological behavior of three sets of ferrofluid-based magnetorheological (MR) fluids is investigated to determine the dependence of the static and dynamic yield stresses, as well as of the magnetoviscous effect on the volume fraction of magnetite nanoparticles phi and of the added Fe particles I broken vertical bar (Fe) , at different values of the magnetic induction B in the MR cell. A wide range of the magnetic particle volume fractions were considered at the two hierarchical levels involved: at the nanometer level, phi = 2.75, 11.67, and 22.90 %, and at the micrometer level, I broken vertical bar (Fe) = 5-40 %. Taking into account also the oleic acid monolayer coating of the magnetite nanoparticles, the hydrodynamic volume fraction of the most concentrated ferrofluid results to be 65 % and, consequently, the highest value of the effective total volume fraction of the investigated extremely bidisperse MR fluids attains 85 %. The diagrams of the static and dynamic (Bingham) yield stresses and of the magnetoviscous effect offer an adequate choice of the nanometer and micrometer range particle volume fractions to fulfill the specific requirements of high-pressure rotating seals and of various MR devices.