Cutting speed limits in towns would increase pollution rather than slow traffic, according to research for a Scottish carbonreduction group.
However, the study also found that lower speeds reduced crashes and noise, and made roads more attractive for walkers and cyclists.
The work, which was commissioned by the universities-based advice group the Edinburgh Centre for Carbon Innovation (ECCI), will be unveiled at a seminar in the capital next week.
Motoring groups welcomed it as the first examination of the environmental impact of changing speed limits, when the previous focus had been on the effect on road safety.
The ECCI said the report, by the authoritative Transport Research Laboratory (TRL), showed there were clear trade-offs involved.
It said: “Reducing urban speed limits has concomitant health, community and safety benefits, but is likely to raise greenhouse gas emissions..”
The TRL report said cutting speeds below 40mph was likely to increase pollution, such as nitrogen oxides and particulates. Carbon dioxide emissions would also go up.
This is because of the increased amount of acceleration and braking in stop-start driving, although these could be reduced if traffic flow was smoothed. The report said 40mph was the optimum speed for minimising vehicle emissions and pollution.
The report found reducing speed limits alone usually cut average speeds by only one quarter of the change in the limit.
The research comes as the A9 Safety Group is considering whether to recommend a trial increase in lorry speeds from 40mph to 50mph on single- carriageway sections between Perth and Inverness.
This would be aimed at reducing long tailbacks behind slow-moving goods vehicles, which are said to have led to frustrated drivers making risky and sometimes fatal overtaking manoeuvres.
Scottish ministers have resisted calls for a similar lorry speed increase across the country, despite their being proposed by the UK government for England and Wales.
The Scottish Government said it also had no plans to raise motorway speeds for cars from 70mph to 80mph, which is also being considered south of the Border.
The Institute of Advanced Motorists said the report laid bare the difficulties thrown up by changing speed limits.
Neil Greig, its Scotland-based policy and research director, said: “We have not previously had an open debate about the impact of speed limits on the environment.
“But when you start to mix speed limits, road safety and emissions, it gets very complicated.”