The 20 miles per hour (mph) zones, proposed as a solution to urban flow and speed concerns, cause far higher CO2 emissions than their 30 mph equivalent, the AA survey, Fuel for Thought, has found.
Since 20 mph zones tend to be adjacent to schools, local authority planners may need to weigh up the conflicting statistics.
Edmund King, president of the AA, says: "Transport and highways planners have little or no official guidance on the environmental impact of 20 mph speed limits.
"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."
The AA also reports that speed humps up fuel consumption by 47 per cent when installed on 30 mph roads.