Since 1988 there is in Portugal a tax called "Imposto Automóvel" (IA - Automobile Tax) that you indirectly have to pay when you buy a new car. The importer pays it to the government and includes it in the final price.
This tax is calculated multiplying the cubic centimeters (displacement) of the car's engine by a value minus a predefined value. You can see the current rates here (page 10).
One of the reasons why this tax is so criticized in Portugal is because IA is subject to Imposto Valor Acrescentado (IVA/VAT - Value Added Tax) at 21%.
When you buy a new car you pay: car's base price + IA + 21% IVA/VAT.
In 2006 the government in functions decided to add an "environment component" (Componente Ambiental) to the tax. It was still called IA (Imposto Automóvel), but 10% of the total tax income should be calculated regarding the car's CO2 emissions.
Next July (2007) this percentage will rise to 30%, and in January 2008 it should represent 60% of the total tax income. The tax name will change from IA to Imposto Sobre Veículos (ISV - Vehicle Tax).
Besides the rise of the environment component, the government decided that part of the tax should be transferred from buying to circulation.
Until now the income from circulation tax, called Imposto Municipal sobre Veículos (Vehicle City Tax), would go to the City where the vehicle was registered. The value of this tax is/was rather low. You can see the current rates here.
The tax due when buying a car (ISV) is expected to drop (average 10%), but the circulation tax, now called Imposto Único de Circulação (IUC - Unique Circulation Tax), payable every year, will rise substantially (average 300-500%). You can see the future rates here (ISV) and here (IUC).
Cars registered before July 2007 will pay IUC according to the current values forever. Cars registered after July 2007 will pay the new values.
Used vehicles imported from any Country pay ISV. When imported from an European Union Country there is a discount given based on age, the older the bigger discount. Starting July imported used vehicles will pay exactly the same IUC that new vehicles pay. This is an "effort" of the government to reduce the import of used vehicles (much more cheaper than new vehicles). You can see the discount percentages here.
If you're moving to Portugal, and you want to bring your car along, you're exempt from ISV. There are three conditions that you must meet to be exempt:
- you must be over 18 years old and have a driver's license for more than 12 months
- you must provide documentation that proves that you were a resident in another country for at least 12 months (a declaration from an official entity - such as an embassy, and some house bills/receipts or income/tax/social security documents)
- you can't sell the car for at least 12 months after portuguese registration
You'll have to visit personally the portuguese customs (Alfândega) and explain your intents. You can find Alfandega's several addresses here.
Sorry, but due to some time restrains, I'm not able to answer any further questions.