Average CO2 emissions of new cars in the UK have fallen by 26.5% since 2000, according to figures from the Society of Motor Manufacturers and Traders.
The average figure was 133.1g/km in 2012, compared with 181g/km at the turn of the century.
European Union targets for average CO2 emissions are set at 130g/km by 2015 and 95g/km by 2020. In 2012, more than 55% of new cars in the UK had emissions that were less than the 2015 target.
If the current rate of decline in average CO2 emissions continues, the UK will meet the 2020 target.
Overall, the reduction comes from improvements in technology, but also drivers spending less money at the pumps because of tough economic conditions and rising fuel duty.
The market share of cars that emit less than 100g/km of CO2 has doubled over the past year to 8.2%.
Alternative fuel vehicles such as hybrids and electric cars achieved a market share of 1.4% in 2012. The Nissan Leaf was the best-selling electric car.