The fuel economy of vehicles is today in everyone's focus. Governments, original equipment manufacturers, and consumers alike are all demanding improvements.Historically, reducing oil viscosity has resulted in improved fuel economy; however, lower viscosities can lead to reduced or “weakened” lubricant films, which may fail to hold up under higher temperatures and heavy loading associated with axle operations. The fluid development challenge is to bridge the gap between fuel economy and operating temperature control.Achieving both fuel economy and durability are not always compatible objectives. The real challenge is to build in the high torque protection historically associated with higher viscosity grades, like SAE 75W-90, while delivering the axle efficiency of lighter grades such as an SAE 75W-85 grade. In previous work, it has been shown that critical factors for achieving the proper efficiency-durability balance include viscosity, traction, fluid film thickness and additive chemistry. [1,2,3,4,5]Axle oils are subjected to different modes of energy dissipation: losses related to loading and losses that are independent of loading i.e., churning or spin losses. In designing versatile axle lubricants, an understanding of fluid rheological properties under both high and low loading is important. High loading is indicative of certain operations such as taxis, while lower loading is consistent with normal city-highway driving.The term durability has many manifestations however here it is used principally to describe a fluid's effect on operating temperature under high speed conditions (as in motorway and autobahn situations). An important consequence of poor or insufficient fluid durability is bearing failure; therefore, bearing life testing has been included in this investigation.The growing need for improved fuel economy is also a global issue due to the relationship between reduced fuel consumption and reduced CO2 emissions.The challenge for vehicle manufacturers is to match the proper fluid with the application to provide the required durability protection while maximizing fuel efficiency.This paper will describe the use of controlled laboratory testing methods for the development of axle fluids that maximize both the fuel efficiency and durability performance across the spectrum of the new viscosity classifications.The relationship of viscosity and fluid formulation choices will be examined with respect to inherent fluid properties, as well as the impact of these fluid properties on axle efficiency and temperature performance characteristics.