The results of an experimental study to measure the tumbling parameter, lambda, for various small-molecule liquid crystals and their mixtures are presented. The methods used include textural observations (twist walls), a direct method, a rheological method, and the oscillatory method developed by Mather, Pearson, and Burghardt in 1995. The single-component results are compared with a molecular theory derived in 1995 by Archer and Larson as well as Kroger and Sellers, which predicts the temperature dependence of lambda, while the results from the binary mixtures are compared to a continuum theory derived by Rey in 1996, giving the concentration dependence of lambda. The results from the four experimental methods agree with each other for single-component liquid crystals, but not for mixtures. This suggests a failure of the single director Leslie-Ericksen theory to describe the rheology of liquid crystal mixtures.