The heightened interest level in Fuel Economy for Heavy Duty Diesel Engines the industry has seen over the last few years continues to be high, and is not likely to change. Lowering the fuel consumption of all internal combustion engines remains a priority for years to come, driven by economic, legislative, and environmental reasons.While it is generally assumed that lower viscosity grade lubricants offer fuel economy benefits, there is a lot of confusion about exactly what drives the fuel economy benefits. Fuel Economy claims in trade literature vary over a broad range and it is difficult for the end user to determine what to expect when a change in lubricant viscosity is adopted for a fleet of vehicles in a certain type of operation. This publication makes an attempt at clarifying a number of these uncertainties with the help of additional engine test data, and more extensive data analysis.In addition, a more extensive explanation is given for the impacts of base oil viscosity index, VI improver shear stability, and blend targets for the finished oil. All of these factors are part of the design parameters of a finished lubricant. With a better understanding of those parameters, lubricant blenders can optimize the viscosity characteristics of their lubricants for improved fuel economy.