Lubricating greases exhibit far more complex tribological behavior than lubricating oils due to their biphasic composition and non-Newtonian rheology. In the hydrodynamic regime, where a full grease film separates the moving surfaces, the tribological response is governed by the rheological dynamics of grease within the contact zone. However, these dynamics are hard to measure directly since the shear rates inside tribocontacts far exceed those accessible on conventional rheometers. To address this, a rheological superposition technique is employed to extend grease flow curves to such extremely high shear rates. The friction‒speed curves for all the grease samples are generated using a ball-on-disc tribometer operating under the isoviscous-rigid regime (low contact pressure and constant temperature). Simultaneously, the variation of electrical contact resistance with speed is recorded. The effective viscosity of each sample in the hydrodynamic lubrication regime is estimated by superposing the hydrodynamic portion of the friction curve onto a master curve. Remarkably, the effective viscosities agree closely with the infinite shear viscosities determined from the superposition method. At high speeds, however, deviations emerge: the effective viscosity falls below the infinite-shear viscosity, indicating a departure from classical hydrodynamic behavior. To our knowledge, this is the first report of a quantitative correlation between grease hydrodynamic lubrication and high-shear rheology. In the boundary regime, the base oil properties, particularly its molecular structure and physicochemical interactions with the contact surface, as well as the morphology and concentration of the thickener, govern the film formation and friction response. These insights offer guidance for formulating greases with tailored properties to optimize lubrication performance.