Research by Godfrey Davis (Contract Hire) showed that emissions from 30,000 vehicles run by its customers had fallen from an average of 190.8g/km a year ago to 184.1g/km this year.
Figures for June show cars registered averaged just 171.2g/km, compared to 181.9g/km for June 2001.
Much of the reduction has come through a massive swing to diesel-fuelled vehicles, with 54.2% of orders specifying a heavy-oil model during June, compared with 34.3% for the same period last year.
John Lyons, GDCH managing director, said: 'New technology has provided company car drivers with the chance to change to more tax-efficient cars, an opportunity they have grasped with both hands.
'Emissions will continue to fall and company car fleets will continue to lead the trend.'
In a double boost for the environment, the new emissions-based tax for benefit-in-kind tax is encouraging drivers to select cars with lower emissions of the greenhouse gas carbon dioxide, at a time when business mileage is falling.
A survey by leasing company Alphabet has found that 90% of the 500 drivers questioned believe their business mileages will decline with the disappearance of the tax will succeed in its aim of tax incentives for high mileage drivers.
However, three-quarters of the drivers said the tax system will have no effect on pollution, despite evidence of reduced CO2 emissions, and the fact that 40% said they would select a less-polluting car in future.
A further 60% simply disagree with the Government's tax policy for company cars.
Mike Baldry, chief operating officer of Alphabet, said: 'It is vital that the Government gets drivers behind its efforts to meet pollution targets. In the long-run, it means lower costs for employers and lower taxes for drivers. But our survey shows the message has not been heard.'