Drivers have been warned not to rely on sat-navs for the speed limit on residential roads in Wales when it is reduced to 20mph from 30mph on Sunday.

Some GPS firms say map updates might not be immediate when Wales becomes the first UK nation to adopt 20mph (32km/h) in built-up areas.

Police in Wales have said enforcing the lower limit will be a "last resort" as drivers adjust to the change.

The RAC said motorists should follow road signs for the speed limit.

"Until sat-nav systems have been fully updated, they shouldn't rely on them to know what the speed limit is on any particular stretch of Welsh road," said Simon Williams, of the RAC.

"It's vitally important that drivers are fully aware of the arrival of the 20mph limit in Wales, and pay full attention to all road signage."

The law will change the speed limit on about 35% of Welsh roads where lamp-posts are no more than 200 yards (183m) apart.

Because it will be the new national speed limit, 20mph signs will no longer be needed on those roads – except when the limit changes.

The UK's first 20mph zone was at Tinsley in Sheffield in 1991, but Wales will follow Spain, who made a similar change to 30km/h (18.6mph) in 2019, and lower the national speed limit nationwide.

Many UK counties, towns and cities now have 20mph limits on residential roads, but Wales will become the first nation to impose it as the default on what the law calls restricted roads – with Scotland still considering whether to follow suit.

Road safety group Brake have called for the 20mph default limit to be extended to England but the UK Conservative government has ruled that out.

Wales' Labour-led government have said their 20mph national speed limit will reduce road collisions, noise and pollution and encourage people to walk or cycle.

Latest official figures show more people were killed or seriously injured in 30mph zones in Wales last year than at any other speed limit.

Of the 1,014 people killed or severely hurt on Welsh roads in 2022, more than 40% of the people (421) were hit in a 30mph stretch of road.

Safety campaigners Brake said if "you're struck by a car travelling at 30mph the risk of dying is five times greater than if the car was travelling at 20mph".

The Welsh government said the £32.5m cost to implement was "outweighed" by reduced impact on the NHS and emergency services, which one study said could save £92m a year.

The Royal Society for the Prevention of Accidents (Rospa) has encouraged wider use of 20mph limits as a "considerably less expensive" way than introducing traffic calming measures.

Of the almost 22,000 miles (35,171 km) of road in Wales, an estimated 7,700 miles (12,500 km) will change from 30mph to 20mph from Sunday.

About 30,000 road signs are set to be replaced – and the roads that will be affected are in a Wales 20mph map produced by the Welsh government.

Ministers think the impact on journey time will be "small" with drivers taking less than one minute extra on average per trip – and with 95% of trips "likely" to be less than two minutes longer.

From Sunday, if you're pulled over doing more than 20mph in a built-up area in Wales you could, in theory, be fined a minimum £100 and get three penalty points.

But police say the response will be proportionate and reasonable while drivers get used to the change as the focus for the first 12 months will be on education – as officers have a level of discretion.

"Our priority is to help people understand why they should slow down and the benefits to their community," said South Wales Police Assistant Chief Constable Mark Travis.

"Where we find people driving above 20mph, we will stop drivers and speak to them about the dangers and risks about driving above the speed limit.

"We won't be taking enforcement action against those people who are actually engaging with us and trying to bring their speed down – so they won't receive a fine.

"The people who will be fined will be those driving at the highest levels. Ultimately, if necessary, we will enforce – but that is always a last resort for us."

While police officers across Wales will initially be advised to use "discretion", all drivers will get fined if they are caught by a fixed speed camera.

The flow of traffic around Wales could change with the introduction of the new limit, especially for drivers using sat-nav to find the quickest route.

Google has told the BBC it is tracking Wales' switch to 20mph, saying: "Our teams regularly monitor for speed limit changes to ensure that we provide the most up-to-date information possible."

Garmin, another GPS firm, said they are working with "external data vendors" to update their mapping but added: "For automotive devices, we update our mapping periodically and for street maps this is up to twice a year."

Not all 30mph roads – about 3% across Wales – are reducing their speed limit because local authorities can and have made exemptions to put the limit back to 30mph.

Councils have been told by the Welsh government that exemptions to their new default speed limit can be made where "strong evidence exists that higher speeds are safe".

Exemption from the 20mph limit can only be given by authorities if they can show pedestrians and cyclists won't be put in danger by higher speeds.

Any roads that are exempt from the new default will have 30mph signs.

According to respondents to a Welsh government-commissioned consultation, more were against the speed limit than supported it while a recent poll found two-thirds of people in Wales didn't want the controversial change.

A petition against the 20mph speed limit was debated in the Welsh Parliament, with the person who started it calling it a "non-democratic whitewash" after having a 20mph pilot area in her north Wales village.

One of the government's own documents said longer journeys could cause a "substantial" economic disadvantage, which for commuters and people who drive for a living meant its "central estimate" was an economic hit of £4.5bn over 30 years.

The Conservatives have called the blanket rollout of 20mph "disastrous, frankly ludicrous and a war on motorists" while the UK government's Commons leader Penny Mordaunt called the new law "insane" and "punishing" motorists on Thursday.

That came 24 hours after the Welsh Tories failed in a last-ditch bid to block the new limit in the Senedd.

The 20mph plan was part of the Labour party's Welsh manifesto when they matched their best-ever Senedd election result in 2021 – and it was also in Plaid Cymru's manifesto too.

The Welsh government said it would "continuously review the impact" of the new limit and First Minister Mark Drakeford admitted it wasn't easy for him to adapt to the lower speed.

"I'm probably not a particularly patient driver," he said.

"I have to make a conscious effort to drive at 20mph but I'm doing it because there's a good reason for doing that.

"I remember when the breathalyser was introduced and how many people appeared to believe that it was quite ok to spend the night in the pub and then to drive – we'd never think of going back to that now.

"This will take time for people to get used to. Once it's bedded in I think people will look back and ask themselves why it was we were prepared to tolerate traffic going at excessive speeds in urban areas.

"It's a small contribution to make five minutes isn't it? When you know that what you were doing and other people are doing will be keeping other people from losing their lives."