The new car tax system is based on measurements of carbon dioxide (CO2) emitted by engines when burning fossil fuels. It is one of a number of gases released as an engine burns fuel, but it is the one that has been linked most closely to global warming.
There are concerns, however, over the accuracy of the figures attributed to each car. According to the Government, the emission ratings for cars will be determined in the model's certificate of conformity, a document supplied with the vehicle.
A spokesman for Volkswagen yesterday queried the reliability of these figures and the ability of manufacturers to adjust the ratings depending on equipment in the car. Weight plays a part in the fuel economy of a car and this can be adjusted by removing various elements, which can then be added on later.
Meanwhile, a senior European car engineer warned that policing these ratings was extremely difficult and that some companies were producing emissions figures that made cars look cleaner than they actually were.
"We have had some genuine efforts within the motor industry to reduce CO2 emissions, such as using narrower tyres, higher gear ratios and reducing aerodynamic factors, and these are very welcome and justified," he said, speaking to The Irish Times prior to the Budget. "But we have also seen some companies putting the base or entry-level model of the car in for assessment and getting a certain low rating.
"What they neglect to take into account is the extra 'heavy' equipment they are providing on the rest of the model range, such as air conditioning, central- locking, wider tyres and sunroofs that make the car heavier and produce more emissions.
"We have even noticed some companies are testing their tyre emissions levels in high temperatures when they will get lower readings. In some cases, manufacturers can bring their car right down from one emissions bracket to a lower one."