The AA has suggested a proposal to cut the maximum speed on motorways would only be a 'cosmetic' change.
The Green Party wants the reduction introduced in order to cut carbon emissions.
The maximum speed on motorways could be cut to 110km/h as part of a review into speed limits.
The party believes the step would improve fuel consumption for those who drive close to the top speed.
But Conor Faughnan, director of consumer affairs with the AA, says it is a "relatively poor idea".
He told Newstalk Breakfast: "I don't think you're looking at any sort of gain - even the environmental gains as mapped out by the Department of Transport are theoretical, not certain if they could be achieved, at most they are tiny and I do mean tiny in the scale of the challenge.
"And yet we do know that the inconvenience and disruption would be quite significant, quite widespread.
"So I think even with a small 'p' political comment, it would be unwise of The Green Party to pursue this agenda.
"The gain just isn't there."
He suggested changes to the car tax system, such as making diesel cheaper, would have more impact.
Recalling that decision that was made the last time The Green Party was in Government, he said: "That was the correct choice at the time on the best environmental information and the latest technology that pertained.
"But the point is that it worked... now that was a genuine environmental measure that made an enormous impact.
"I think this one would be a cosmetic environmental measure: it's impact would be felt much more by the punter than any environmental gain".
Green Party MEP Ciarán Cuffe admits it would take longer to get from 'A' to 'B'.
"I think there's no perfect measure to reduce our carbon emissions, and whether it be agriculture or transport or energy or construction, there will always be both up sides and down sides to any measure that is proposed".
"But we do know our emissions have been going up, we do know that we need to reduce our emissions by over 7% a year
"And a small reduction in the speed limit from 120 down to 110 kilometres on motorways - it might mean extra travel time of perhaps eight or nine minutes on a 200k trip, but it could lead to a 10% reduction in emissions.
"And in overall terms, it could lead to a 1% reduction in Ireland's emissions, which would be an extraordinary step in the right direction.
"Speed limits are never easy and getting the speed limits right it has to be a balance between mobility, safety and the environment.
"But in this instance, I would argue that the improvement for the environment is worth doing".
But he added: "There's no quick fix on climate change, and I think Fianna Fáil and Fine Gael have to step up to the mark and have a rational debate about what measures are the way to go".
