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".

The measures are outlined in the Digital Economy Bill, which was published on Tuesday.
It says phone operators could be penalised if they miss coverage targets and other licence commitments made to the sector's regulator.
And it sets out how a separate watchdog could punish adult sites that fail to check their visitors' ages.
Other measures include:
If the bill is passed, Ofcom would be able to fine mobile networks a maximum £20,000 a day, up to a total sum of £2m, if they fail to hit agreed service goals.
The regulator has suggested this would give it "more flexibility in our important work to manage and ensure optimal use of spectrum".
"At the moment, when a provider breaches its licence, in most cases our only sanction is to remove the licence completely, or launch criminal proceedings," a spokesman said.
"The bill would allow Ofcom alternatively to impose financial penalties, which may in some cases be a more proportionate means of securing compliance."
The law also proposes creating a separate age-verification regulator, which will be told to publish guidelines about how sites should ensure their users are aged 18 or older.
It says this body should be able to fine pornographers up to £250,000 if they fail to comply.
Details of how this would work in practice have still to be determined.
But the bill states that the watchdog would be able to alert credit cards and other payment providers as well as other companies providing a service to the sites in cases where it cannot persuade their owners to comply.
One detail not mentioned in the bill is that the new broadband universal service obligation (USO) should set 10Mbps as the minimum download speed.
However, a factsheet issued by the Department for Culture, Media and Sport does refer to this as being its "ambition".
"Ofcom has reported that 10Mbps is the speed needed for a USO, given a typical household's use of digital services, and is adequate for demanding services like streaming high definition video content," it says.
"The speed will be specified in regulations to allow it to be updated when necessary.
"Regulations can be updated in a much shorter timescale than a new bill, which can take a year before it can be brought into force."
Assuming there are no major amendments to the bill, one industry watcher said its impact would probably be determined by how tough the two regulators were willing to be.
"No matter how the law gets written, it's how the regulators implement it and who they chase for non-compliance that determines its real-world effect," said Andrew Ferguson, from the Thinkbroadband news site.
"Ofcom has a history of being a light-touch regulator - it tends to concentrate on market failures rather than trying to drive the whole market forward."
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.
[NEXT_CONCEPT]
The government has proposed new powers to fine mobile network operators and pornographic website owners.