A number of dealerships selling new cars are refusing to say over the phone how much their vehicles cost under the new car registration tax scheme, two weeks after changes announced in the budget.

The Sunday Times phoned different agents to see how the prices of their cars will change once the new regulations are in place in January.

Several salesmen were coy about the new prices and even asked to meet to discuss "the best options". A number refused outright to give the prices over the phone.

The government announced a new registration tax and circulation tax, replacing the vehicle licence, in a bid to encourage the purchase of new, smaller and environmentally-friendly cars.

Yet analysis shows that the biggest cuts will apply to diesel-engine cars, despite a special particulate matter tax being imposed on them.

In some cases this means there will be little difference in price between petrol and diesel versions of the same model. On the other hand, eco-friendly hybrid cars will become more expensive across the board.

The government, however, has made it clear it will be tweaking the tax, in particular for diesel family cars.

Our analysis also reveals that cars within the same price bracket will be reducing their prices at different rates. Whereas a Ford KA will become 13.2 per cent cheaper, the price of a Daihatsu Sirion will be reduced by 7.8 per cent.

The same can be said for the cars in the 1.4 litre petrol bracket, where a Citroen C4 will be reduced by over €3,000 while the price of a Fiat Bravo will be trimmed by €1,050.

The car registration tax is worked out through a complex system taking into account the car's length and value, the combined CO2 emission level and, for diesel cars, its particulate matter emissions.

The prices of cars with engines with a capacity of less than 2.5 litres will all be reduced, those with the smallest engines by 44 per cent and those between two litre and 2.5 litre by five per cent.

Environmentalists have criticised the government for only penalising the cars over 2.5 litres. They say all cars above 1.6 litres should be discouraged.