Adding speed humps to a 30 mph road, meanwhile, can boost CO2 emissions by 46.9%.
The first piece of research for the AA's Fuel for Thought campaign claims that on average, petrol car fuel consumption on longer and relatively free-flowing 20 mph urban streets can worsen by 5.8 miles per gallon (1.3 miles/litre).
Over a year, a petrol car commuting along such a road could generate nearly a quarter of a tonne of additional CO2 every quarter of a mile.
"Transport and highways planners have little or no official guidance on the environmental impact of 20 mph speed limits," said AA president Edmund King.
"It would be a bitter and unpalatable irony if local authorities, that have targeted owners of larger vehicles with environmental charges, are found guilty of pumping up CO2 emissions through indiscriminate use of 20 mph restrictions."
King added that the AA accepted that targeted 20 mph speed limits in residential areas were popular and improved safety. Along shorter roads with junctions and roundabouts, limiting acceleration to up to 20 mph reduces fuel consumption.
However, he said a 30 mph limit on local distributor roads may be more environmentally-friendly.
"The Green Party has been advocating 20 mph limits across the whole of London, perhaps without realising that this policy would backfire in terms of environmental emissions," added King.
"We need independent research to ascertain both the safety and environmental implications of 20 mph zones so that authorities don't make a huge and widespread environmental mistake. Researched guidance on 30 mph versus 20 mph limits versus speed humps will help road engineers to make informed decisions on where best to site lower speed restrictions on urban roads."
Green Mayor of London candidate Siân Berry dismissed the AA study as 'wilful misinterpretation' because it looked at cars' fuel consumption at steady speeds of 30 mph and 20 mph on a test track, not under conditions even approximating city driving.
"Driving around a test track in scientific experiments has nothing to do with the stop-start reality of city motoring," said Berry.
"It's really disappointing to see a normally reasonable organisation deliberately standing in the way of road safety, efficient transport and the health of the environment.
"Everyone who actually drives in London knows that you very rarely spend more than a few seconds at 30 mph. By lowering the maximum speed, we can reduce the stop-start nature of London driving. There's will make our roads safer, reduce pollution and may even make journey times quicker."