By Paul Rincon
BBC News science reporter
European car makers are defaulting on a vital target to tackle climate change, according to an environmental group.
The motor industry is responsible for 15% of the EU's CO2 emissions
Their efforts to boost fuel efficiency are falling "far short" of a commitment made to the European Union in 1998, Transport & Environment says.
The more fuel a car uses, the more carbon dioxide - a key greenhouse gas - is emitted into the atmosphere.
But motor manufacturers say they are already working harder than many other industries to cut emissions.
In 1998, the European Automobile Manufacturers Association (Acea) agreed with the European Commission an average emissions target of 140g of CO2 per kilometre for new cars by 2008.
Japanese and Korean manufacturers, which command a much smaller part of the European car market, have pledged to meet the same target. But they have an extra year to do so.
Last year, European manufacturers sold cars that produced on average 160g of carbon dioxide (CO2) per kilometre.
This was down 1% on the previous year, according to sales figures analysed by Transport and Environment (T&E).
T&E says car makers will now need an improvement rate of 4.3% per year over the next three years to meet their commitment. To date, T&E says, the best performance was 2.9%, recorded in 2000.
Jos Dings, director of T&E, said car makers "put all their technology into making cars heavier and more powerful, rather than more fuel efficient".
A spokesman for the UK Society of Motor Manufacturers and Traders (SMMT) told the BBC News website: "The 140g target was always going to be a very challenging target and it's not a manufacturer-only issue.
"In the UK, we've made a 10.7% improvement in CO2 emissions since 1997. We're all making steady steps forward in refining engine technology."
Aat Peterse, programme manager for clean cars at T&E, said by using lighter materials and making adjustments to performance, manufacturers could make further improvements to efficiency at relatively little expense.
"You can economise a bit on top speed; instead of 250km/h (155mph) you can stop at 220 (135mph). You can economise a bit on acceleration; instead of six seconds to 100km/h (62mph), you can make it 6.5 or seven," he told the BBC News website.
The UK SMMT counters that "cross-legislative" factors complicate the issue.
"There are a whole range of measures we have to put into cars related to safety, such as side impact bars and airbags, as well as all the things people want for comfort such as aircon and sat-nav," a spokesman explained.
"All those things add to the car's weight. You've got to find a way of moving that weight at the same speed, which of course means you need more 'oomph'."
Mr Peterse responded: "They have a point there, but I find it hard to say to what extent. We know that cars are getting heavier every year by 16 kilos (35lbs), and you can look at cars in the street and see them getting bigger.
"They think that's the way to add value - to make cars bigger and more comfortable and more powerful and raise their margin on the car. I would say that is the major source of this phenomenon."
T&E is pushing for regulation of the industry to control CO2 emissions from cars in the same year that the European Commission is reviewing its climate policy on cars.
"The Commission has decided to rely on the pledge from industry to bring down average emission levels; but it is clear now the industry is not making good on that promise. Therefore, it is time for the Commission to stop relying on that pledge and move forward with regulation," said Mr Peterse.
But the SMMT said it considered self-regulation the best option: "A flexible approach... is the way we as industry think it should work and has worked quite successfully," a spokesman said.
"It would be difficult to work to in the long term. We are a global industry and we have to remain flexible."
Acea was not immediately available for comment.