An undercover investigation by BBC NI has exposed a growing problem with mileage fraud.
Reporters with secret cameras posed as car buyers to gather evidence of how unscrupulous dealers operate the mileage fraud.
Trading Standards has confirmed that car clocking is on the increase.
"We would be hard pushed to identify a town or city where there isn't a clocker or a number of clockers operating," Trading Standards said.
The profits from clocking can be substantial and there is evidence unscrupulous dealers and even members of the public are winding cars back by as much as 100,000 miles.
BBC Consumer correspondent Martin Cassidy said the rules of valuing a second hand car are simple.
"The value of a vehicle is based on the make and model as well as the year of manufacture but condition and mileage are also significant factors," he said.
Mr Cassidy said the evidence was that for an increasing number of cars in Northern Ireland, the mileage you see is not the mileage the vehicle has actually done.
Justin Gawn, a first time buyer from Bangor was one of the victims of the car clockers.
He said he thought he was buying a car that had travelled just over 86,000 miles.
But despite having made checks, when he actually purchased the vehicle, his suspicions were aroused and he began to look into the car's history.
It turned out that Justin's car had actually travelled in excess of 100,000 miles and he had to embark on a lengthy legal battle for compensation.
His story is by no means unique, similar complaints have resulted in a string of prosecutions against the clockers.
The authorities say mileage fraud is becoming a major problem and the BBC investigation paid a visit to a car dealer who has been prosecuted for clocking.
Armed with secret cameras, two reporters went to have a look at Seven Towers Autos near Ballymena.
The Peugeot they asked to test drive was advertised on the internet with a guaranteed mileage of 117,000.
That was the mileage displayed on the dashboard and confirmed by a salesman.
But official records told a different story.
According to the Driver and Vehicle Licensing Authority, the Peugeot had an MOT test in April 2010 when the mileage was recorded at over 182,000 miles.
Somewhere in the intervening time the Peugeot car had unaccountably lost around 65,000 miles.
Seven Towers Autos declined to be interviewed by the BBC but claimed the mileage problem would have been spotted at the point of sale.
Despite the introduction of digital mileage displays and MOT mileage records, clockers continue to operate.
While older mileage displays could be wound back mechanically, the newer digital displays can be altered by a whole array of gadgets which are available on the internet for a little over £100.
Anyone who has encountered a problem with car clocking should contact Trading Standards.