Cars registered pre-2008 are taxed on the basis of engine capacity. Licensing authorities will amend the engine capacity of the vehicle, where there has been a change of engine, if the number and cubic capacity of the new engine have been certified by a main dealer, and motor tax liability is adjusted accordingly where appropriate.
In the case of cars registered since 2008, the basis of taxation is CO2 emissions, as provided on the Certificate of Conformity for the vehicle. The level of CO2 is determined at manufacturing stage as part of the type approval process for the vehicle. The level of CO2 is the level emitted in terms of grams per kilometre driven and is affected not just by engine size, but also by factors such as body type and weight, aerodynamics, type of transmission, etc. I understand that determining the level of CO2 emitted by a vehicle is expensive and that it is generally only carried out in respect of new vehicles at manufacturing stage in facilities in mainland Europe. I do not, therefore, propose to provide for a variation of motor tax in respect of such modified vehicles.