Three people, including two from Lancashire, are facing prison for their parts in a £500,000 “clocking” scam which is said to have ripped off car and van buyers across the UK.
The fraud involved more than 40 vehicles which had their mileage readings turned back.
Buyers would travel to Darlington after seeing adverts in AutoTrader magazine.
Police discovered the high mileage vehicles were being bought from auctions and leasing companies and then having their mileage reduced, a court in Teeside was told.
Operation Sledgehammer was launched by police after complaints to consumer watchdogs from as far afield as Penzance and Scotland.
Alfred James Welch, 59, of Whinfield Road, Darlington, and his sons, Alfred Welch, 32, of Scale Hall Lane in Lancaster, and James Welch, 27, from Morecambe, were among those arrested.
Welch Snr pleaded guilty to conspiracy to commit fraud at an earlier hearing.
His sons admitted the charge when they were due to go on trial on Monday.
Charges against other defendants were dropped however.
The three who pleaded guilty will be sentenced at Teesside Crown Court in April.
Some 80 police officers from Durham Police and Lancashire Police took part in raids last February as part of the Operation Sledgehammer operation.