The number of pedestrians and cyclists hurt in accidents fell by more than a third between 2012 and 2016 - but the rate was lower in 20mph zones, the city council said.
The council said it will now evaluate the effectiveness of existing zones.
It also agreed to spend the Â£687,000 road safety pot on alternative schemes.
Since 2014, a 20mph limit has been introduced on more than 1,000 roads and around 138 schools in the city.
This has cost the council about Â£640,000, according to a BBC Freedom of Information request.
It led to an average speed reduction of 0.7mph in these areas but the "amount of accidents experienced in 20mph zones has not fallen as quickly as initially hoped", the council said.
For example, casualties of cyclists on the city's roads fell by more than 40% between 2012 and 2016 but the drop in 20mph zones was notably lower - 16% in Gorton and 12% in Miles Platting, Newton Heath, Moss Side and Fallowfield.
Last year, one person in Greater Manchester was convicted for exceeding the speed limit in a 20mph zone, police said.
While the review is undertaken, money for road safety will be spent on other traffic calming measures and pedestrian crossings.
Councillor Rosa Battle said: "It's important that we fully understand how effective these zones have been in terms of reducing accidents so far."
Rod King, director of 20's Plenty for Us, welcomed the report but said it was "incorrect to use early, small number datasets which are not statistically valid and compare them to whole city totals".
A plan to expand 20mph zones across Manchester is to be reviewed after their impact on reducing the number of accidents was called into question.