The reduction in CO2 emissions is thanks to fleets leasing greener cars and demanding that their drivers cover fewer annual business miles.
As a result, British fleets have cut their carbon footprint by three million tonnes over the past three years.
According to figures from the British Vehicle Rental and Leasing Association (BVRLA), this represent the equivalent of one-and-a-half tonnes of CO2 saved for every car run by its members.
“Our members and their customers are doing more than just choosing lower polluting cars…they’re covering fewer miles.” said the BVRLA’s director general, John Lewis.
“Average mileages have reduced from more than 25,000 miles per annum to 21,643 miles last year.
"Put these two areas together and this is a resounding success in our commitment to reducing greenhouse gas emissions in the fleet sector.”
The figures were obtained from a survey of fleets carried out at the end of 2007.
From this the BVRLA discovered that average CO2 emissions have reduced from 171.8g/km in 2003 to 157.4g/km in 2007.
This outstrips the decreases across new car registrations as a whole.
According to the Society of Motor Manufacturers and Traders (SMMT), in 2007 average new car CO2 figure dropped 1.4% year-on-year to 164.9g/km, a fall of 13.1% since 1997.
The SMMT claims that improvements in vehicle technology have seen savings of around one million tonnes per year across the whole new vehicle parc of fleet and private registrations.
This figure, compared alongside the BVRLA’s research, suggests that almost all the CO2 savings are coming through the registrations of fleet vehicles, and that the effect on private buyers of tax incentives such as graduated VED are less effective than those incentives in the fleet market.
Carbon emissions in the fleet sector have dropped steadily since the introduction of CO2-based company car tax.
With Class 1A National Insurance Contributions also related to CO2, there are now fiscal incentives for drivers and employers to choose lower emission cars.
Alongside tax, the high price of fuel is also concentrating fleet minds.
Also unknown is the effect of schemes such as the London congestion charge on driver choice.
No decision has yet been made on whether there will be a 100% discount for sub 120g/km cars entering the zone, although an announcement is likely any day.
There is still some way to go before CO2 emissions are at a level acceptable to the legislators.
If reductions continued at the current rate by 2012, average CO2 emissions for fleet would total 142g/km, still some way off the target of 125g/km average for all new cars by 2015 that the European Parliament is keen to introduce.
Certainly there is little chance of the retail new car market hitting those figures but the fleet sector could well be the first to achieve it.
Many manufacturers have yet to introduce fuel and emissions saving technology across their entire ranges, instead offering only a small selection of ‘eco-friendly’ models, meaning low emission choices are still relatively limited.
If all manufacturers managed to make the leap in emissions reduction that BMW made last year with the introduction of its EfficientDynamics technology, then it is conceivable that CO2 savings could accelerate noticeably.
Key fleet models such as the BMW 320d saw CO2 emissions fall 16% to 128g/km.
Should other major manufacturers manage to make such savings in the next three to four years across their ranges then the UK fleet industry, with its predilection for low CO2 diesel cars, could find itself way ahead of any other sector in Europe for running cleaner cars.
But official figures do not tell the whole story.
As a spokesman for the SMMT says: “What benefit would ultra-low low CO2 cars present, if drivers spent twice as long in traffic jams with engines running?”
Britain has the most congested roads in Europe with business users also averaging the longest commute, and technology such as stop-start systems and tyre pressure monitors could save thousands of tonnes of CO2 in real terms, especially in slow moving traffic where engine run inefficiently.
It has been suggested by many tyre experts that running on underinflated tyres can harm fuel consumption by up to 10% - negating any technological improvement fitted to new cars.
And the BVRLA has found that fleets seem to be a covering fewer miles, which it puts down to a combination of factors: the high cost of fuel, congestion and firms becoming more aware of their corporate social responsibility, through cutting the burden on high mileage drivers and reducing their carbon footprint through the use of home working and employing more advanced communication technology.
“Our members and their customers are doing more than just choosing lower polluting cars,” said John Lewis.
“They’re covering fewer miles.
"Average mileages have reduced from more than 25,000 miles per annum to 21,643 miles last year.
“Put these two areas together and this is a resounding success in our commitment to reducing greenhouse gas emissions in the fleet sector.”