The higher 110kph speed limit on the new Waikato Expressway is expected to boost carbon emissions by more than 4300 tonnes a year.
The new limit, up from 100kph from July 13, will save up to 4 minutes and 20 seconds in travel time if driven for the entire 78km of the expressway.
The transport agency Waka Kotahi said it had modelled the environmental impact of the increased speed limit before deciding to introduce it.
Two academics who specialise in climate policy and transport have criticised the emission-boosting decision as heading in the wrong direction when transport emissions must fall.