Top 20 Least Polluting Vehicles of 200920th place (tie): Saturn Astra, Ford Focus, Mazda3, Pontiac G5, Nissan VersaA number of cars post a CO2 per year rating of 3,552 kg. The Saturn Astra with a manual transmission, the Ford Focus with an automatic transmission (which should get better with the double-clutch model in 2010), the 2.0-liter Mazda3 with a manual transmission (the only Mazda on the list), the Pontiac G5 with an automatic transmission and the Nissan Versa with an automatic transmission.19th place (tie): Chevrolet Cobalt and Pontiac G5 (manual)Equipped with a manual transmission, the Chevrolet Cobalt and Pontiac G5 improve their score to 3,504 kg/year of CO2.19th place (tie): Hyundai Elantra (manual)The Hyundai Elantra with a manual transmission also releases 3,504 kg of CO2 per...