A government website set up to help motorists check whether they will face charges for entering Birmingham's clean air zone (CAZ) may be giving people false information, the RAC has warned.
The guide is advising drivers ahead of the zone's July launch.
The RAC says it may show vehicles as not complying in Birmingham, when the same ones would in London, despite the cities having the same requirements.
The Department for Transport said it was ironing out issues on the new site.
"The website has only just been released and we are learning from customer feedback," a spokesperson said.
"It's very helpful that this has been brought to our attention early and we're working quickly to iron out these teething problems."
Birmingham's CAZ is designed to crack down on vehicles deemed high polluters in the city centre and will be overseen by the city council.
The government's guide to charging went live on the Department for Transport's (DfT) website on Wednesday, and asks drivers to enter their vehicle's registration number to see how they will be affected.
Rod Dennis, a spokesman for the RAC, warns the website will show as non-compliant some vehicles which meet emissions standards for the zone.
This, he says, is despite Transport for London's online checker for its Ultra Low Emission Zone showing the same vehicle as compliant.
Birmingham and London have the same requirements in relation to diesels, according to Mr Dennis.
He said: "A number plate is not a reliable way of finding out a car's Euro emissions standards, so we recommend drivers double check with their vehicle manufacturer to be sure.
The government's online guide tells users that accuracy "is always improving" and urges motorists to recheck their vehicle's status shortly before driving into a zone.
Birmingham's chargeable area is within the A4540 ring road, but not on the road itself, with proposed charges for high-polluting vehicles at £8 for cars and £50 for buses and HGVs.
The city council said the checker site also had a "dedicated help desk" to deal with inquiries about vehicle compliance.
Follow BBC West Midlands on Facebook, on Twitter, and sign up for local news updates direct to your phone.