Average emissions from new cars in the EU fell by nearly 4 percent in 2010 from 2009, according to data collected by the European Environment Agency (EEA), an arm of the European Union.
Many manufacturers were already on track to meet 2012 green targets.
But Daimler AG, Honda Motor Co, Nissan International SA, General Motors Company, Mazda Motor Corporation and Dacia, of the bigger companies, would all have to cut average emissions over the next five years by more than 14 grammes of CO2 per kilometre.
The EEA said that without further progress, penalties could total 10 billion euros, which could add to the cost of new cars.
The latest available figures for 2010 showed the average level of emissions of a new car registered in the European Union was 140.3 grammes of carbon dioxide per km.
As part of EU goals to reduce the CO2 emissions, car manufacturers must achieve an emissions target of 130 grammes per km by 2015 on average for new cars registered in the EU.
This target will be gradually phased in from next year.
Specific emissions goals are assigned to each car manufacturer, depending on the average mass of the fleet.
Fines for failing to meet the target will be calculated on a progressive scale for each additional gramme of CO2 above the target, multiplied by the number of cars sold.
Thirty-two manufacturers, representing almost 80 percent of 2010 registrations in the EU, achieved their 2012 specific emissions targets two years in advance.
Among those that have already met 2012 targets is Toyota Motor Europe, with emissions less than 1 gramme of CO2/km away from a tougher 2015 target.
Peugeot as well as its Citroen arm were also close to reaching their 2015 targets already.
Three manufacturers produce only electric vehicles, and their emissions are listed as zero.
Of those producing some conventional-fuelled cars, Maruti Suzuki India had the lowest CO2 emission level overall (104 grammes of CO2 per km) of manufacturers registering vehicles in Europe.
Among the larger manufacturers, Fiat Group Automobiles Spa had the lowest average CO2 emissions in 2010 (125 grammes/km).