The Society of Motor Manufacturers and Traders (SMMT) reckons industry CO2 savings far exceed those projected for London's congestion charge extension and has called for an extension to the mayor's consultation on the proposed changes.

The SMMT claimed that improvements at UK car and commercial vehicle manufacturing sites have cut CO2 from 2.14 to 1.36m tonnes in just four years, a saving of 36.5%. Average new car CO2 emissions have also come down by 12% in a decade, saving an estimated 1m tonnes of CO2 each year in the UK.

TfL's own figures claim a CO2 saving of up to 8,100 tonnes under the plan to base central London charges on car CO2 emissions. According to SMMT, that compares to total ground-based transport emissions in London of 9.7m tonnes. In other words, the maximum benefit for the capital would be a CO2 reduction of just 0.084%.

In an initial response to Transport for London (TfL), the motor industry body said current proposals would neither significantly cut CO2 nor reduce congestion in the capital and that, given the complexity of the scheme, more time is needed for discussion.

Concerns about environmental improvements come after an independent report suggested that changes could encourage between 4-10,000 additional cars onto central London roads. That means more congestion and delays for drivers within the zone, as well as minimal benefit for the environment, if the plans proceed, the SMMT said.

"The motor industry has asked the mayor for an extension to the consultation period to work with Transport for London," said SMMT chief executive Christopher Macgowan.

"The issues are complex and TfL must be absolutely clear about the scheme's aims. Its execution must also deliver the greatest benefit both in terms of congestion and CO2 reduction and the charges to drivers must be proportionate."