The number of road tax evasions has hit its highest level in Northern Ireland in four years.
Almost 30,000 cases were detected in eight months last year, the Driver and Vehicle Licensing Agency (DVLA) said.
Paper tax discs were scrapped in 2014 and the AA said drivers are still unsure of the new process.
Jack Cousens, head of roads policy for the AA, said: "The increase in Northern Irish drivers evading road tax over the past four years is concerning.
"More than three years on from the changes to road tax, it seems drivers are still unsure of the process when buying cars privately.
"In the past the vehicle would be sold with the remaining tax intact, but now the onus is on the buyer to tax the vehicle before they drive away with their new purchase."
The DVLA responded to an freedom of information request.
From July 2014 to the following March, 26,424 cases were created after reports of unlicensed vehicles on the road in Northern Ireland.
Reports were received from the DVLA's wheel-clamping contractor and automatic number plate recognition (ANPR) vehicles, and also from police and local authorities.
In 2015/16 the total had fallen to 17,230 but the following year had bounced back to 24,745.
From April to November last year the tally stood at 27,018.
Mr Cousens said drivers needed to ensure they kept records up to date and advised the DVLA of any change of address, to ensure they continued to receive official reminders.
He said many drivers held on to old tax discs as a reminder for when tax and MOT is due on their cars.
"For some drivers there is clearly still some affection for a little circle of paper on the windscreen, but it is probably too early to call for the return of the tax disc."