The composition and size of the passenger car fleet is often influenced by legislation and policy, including taxation, scrappage, and vehicle registration policies. Numerous measures at national and international level have been investigated to reduce the air pollution and/or climate change impacts of passenger fleets. However, the objectives of climate change and air pollution policy occasionally are conflicting, and this has resulted in a number of well publicised policy shortcomings. This paper outlines the development of a fleet emissions control (FEC) framework which enables the dynamic and adaptive optimisation of fleet emissions through changes in taxation policy in Ireland. The data for analysis was obtained from national sources including datasets from the national emission inventory and COPERT model. Applications of regression-based modelling and statistical analysis were conducted to predict fleet, mileage, and emissions changes. The results of the FEC framework showed that no increase in NOx emissions, above 2007 levels, could be achieved at a cost of a small increase in CO2 in 2016 (+1.8%). This could be achieved by taxation policy primarily resulting in a shift away from small engine diesel vehicles (<1.4 L) in the fleet which were responsible for the largest contribution to emissions. In the transition of vehicle fleets to full electrification or full decarbonisation, the FEC framework developed here offers the potential to formulate practical and optimised adjustments in static taxation systems. These adjustments will assist in achieving the requirements of national policies on reducing emission.