Tests were conducted throughout the temperature range of -65-0 F with a truck axle equipped with observation windows and temperature-measuring equipment. Twenty-one test lubricants encompassing a wide range of physical properties were employed in the investigation. Based upon this study, the flow characteristics of satisfactory sub-zero gear lubricants have been identified. Correlations of the truck axle flow results with several commonly specified tests for fluidity were attempted and discussed. The development of a small axle flow simulator is described, and its correlations with full-scale axle flow results are discussed.