I have no plans to bring in legislation to change the basis for charging motor tax for vehicles registered prior to 2008 to the CO2 system. The question of applying the new CO2 based motor tax system to all cars was subject to detailed discussion during the debate on the Motor Vehicle (Duties and Licences) Act 2008, when the CO2 system was being introduced. Any retrospection would not be possible, as the CO2 data for the pre-2008 car fleet have not been authenticated for business purposes. The cost of any retrospection could be substantial were it possible, depending on how it applied, and could have direct implications for the revenue base of local government. Any loss in revenue would have to be compensated for elsewhere in the motor tax system, or through the taxation system generally.