Three empirical rules for prediction of the shear rate dependence of viscosity and normal stress from linear viscoelastic response are compared with the dependences for a fluid described by the Wagner model. The linear viscoelastic relaxation spectrum used gives a reasonable form for the functional dependence of the viscosity upon shear rate. Further, the integral required for the Bersted approximation can be obtained analytically for this spectrum. The result is a viscosity‐shear rate relation of a form proposed empirically by Bueche and Harding, and which is numerically equivalent to the predictions of the Graessley entanglement model. Over a wide range of power‐law slopes and of shear rates the agreement of the Wagner model and of the three empirical rules is excellent. The reason for the agreement is that for all these rules the rheological functions are computed as integrals over a broad, shallow sloping relaxation spectrum which is truncated by a relatively steep cutoff function. The integrals are therefore not very sensitive to the detailed form of the truncation function.