15 Mar 04
The price of your annual road tax disc is based on how much carbon dioxide (CO2) your car emits and what fuel it uses. The old system, which was based only on engine size, was changed to encourage drivers out of gas-guzzling, high-emissions older cars and into newer, smaller-engined ones.
The Government hopes that the CO2-based system will encourage the purchase of more fuel-efficient vehicles and those emitting less carbon dioxide (CO2 output is proportionate to the amount of fuel burned), and also to get drivers to use more environmentally-friendly fuels.
There are six CO2 grades. No new cars on sale currently meet the criteria for the lowest Band A (emitting up to 100g/km) bar special-order electric models, but a number of small city cars, and larger hybrids such as the Honda Civic IMA and Prius, qualify for Band B (101-120g/km). Inside these six bands, there are additional penalties or incentives based on the type of fuel used: alternative fuels get a part-rebate, while diesel fuels are penalised for their increased particulate emissions.

If your car was registered before 1st March 2001, then the Vehicle Excise Duty you have to pay is based on your engine size.
A full listing is available at the DVLA Website.