Excluding niche brands such as Smart and Hummer, the figures from cleangreencars.co.uk showed Fiat and Mini topping the chart with average CO2 emissions of 138.2 and 139.6g/km respectively.
At the other end of the spectrum is Porsche with CO2 output at 275.6g/km.
According to Clean Green Cars, Porsche’s emissions went up slightly in the first half of 2008.
A decade ago, carmakers agreed voluntarily to achieve average CO2 emissions under 140g/km.
These figures show how far most manufacturers have fallen short, said the green motoring website.