Intrinsic crystal slip system properties which control stress, stress relaxation and strain rate sensitivity (SRS) are investigated using combined experimental bend testing, digital image correlation strain measurement and electron backscatter detection with crystal plasticity modelling. Intrinsic properties (activation energy and volume) have been determined from room temperature experiments on Zircaloy-4 which has been shown to creep at 20 °C with non-negligible SRS of ∼0.03. Polycrystal plasticity modelling has been shown to capture DIC-measured intra- and transgranular creep strain and SRS evolution, and intragranular stresses have been calculated and shown to quantitatively reproduce those estimated from differential DIC strain measurements. The modelling also reproduces polycrystal sample-measured stress, stress relaxation and SRS behaviour remarkably well demonstrating consistency from slip system properties to polycrystal strain rate sensitivity. Polycrystal plasticity finally predicts diffraction-measured effects of texture and slip system activation on lattice strain rate sensitivities.