The invention is a device for controlling the temperature and thus the viscosity of the lubricating fluid in a rear axle or differential. A first embodiment of the device comprehends a curved bi-metal strip or plate disposed proximate the ring gear and closely conforming to it. The strip extends about the periphery of the ring gear in the direction of rotation of the gear when the vehicle is moving forward. As the temperature of the lubricating fluid increases or decreases, the bi-metal strip or plate moves to direct a larger or smaller flow of the lubricating fluid toward the housing through which heat is transferred to the atmosphere. In another embodiment, a bi-metal baffle disposed adjacent the ring gear includes a plurality of flaps which open upon a rise in lubricating fluid temperature and direct more fluid to the housing to assist heat dissipation.