This thesis presents the research carried out to develop and optimise a novel aerodynamic add-on device for commercial vehicles which would reduce overall fuel consumption without any impact of vehicle aerodynamic stability. A prototype of such a device has been manufactured with existing legal and operational restrictions in view. Additionally, effects of such a device on the rollover stability of a commercial vehicle have been investigated and quantified. This has been done by developing a set of analytical methods to first quantify aerodynamic forces in terms of vehicle shape and flow conditions and then to predict the stability of road vehicles, both using mathematical models formulated using available data.