This article investigates the rheological and mechanical properties for blends of recycled high-density polyethylene (HDPE) and virgin polyolefins and attempts to correlate relative shear viscosity and relative stiffness for these blends. These virgin polyolefins comprised a wide variety of flow characteristics, from high-flow injection molding, low-density and linear low-density polyethylene to very low-flow film blowing grade high-density polyethylene. It can be seen that there is a variety of behaviors for the relative viscosity and relative stiffness of the blends studied. Relative viscosity and relative stiffness can largely be described by linear curves. This article categorizes these parameters according to the gradient of these linear curves. The difference between the relative viscosity gradient and relative stiffness gradient is identified as a product of a variety of factors, including branching content, viscosity level, and the nature of any side units.