This paper presents the results of an ex post evaluation of the impacts of a vehicle tax reform in Ireland, by carrying out a full social cost benefit analysis of a vehicle tax reform that began in Ireland in 2008 and shows that whilst successful in improving the fuel economy of new passenger cars, it may also have caused unintended effects, such as an increased proliferation of diesel vehicles in the passenger car fleet. These outcomes have mitigated the overall benefits. In addition to quantifying the scale of the various effects and outcomes, this paper clearly demonstrates the importance of broad scope policy design.