Bentonite is commonly used as a drilling fluid additive for viscosity control during drilling of oil wells. Rheological behavior of bentonite suspensions is very complex and it is very difficult to predict accurate shear rates from viscometer readings. This paper deals with the accurate estimation of shear rates of bentonite suspensions using rotating coaxial-cylinder Fann viscometer readings. Rotor rotations and bob deflections are the two important Fann viscometer readings which are used to predict shear rates with the help of generalized viscometer difference equation. Rheological behavior of bentonite suspensions are also compared with classified fly ash suspensions with varying particle size and solid loading. The proposed shear rate equation is quite general and can be applied to any fluid. (C) 2013 Elsevier B.V. All rights reserved.