Last year saw an unexpected increase in the number of vehicles found with mileage discrepancies, indicating a trend of more sellers "clocking" cars, according to vehicle history expert HPI.
There may be up to 486,000 vehicles with a false mileage on UK roads, based on the SMMT's used car sales for last year.
These "shocking" figures show a worrying trend according to Shane Teskey, senior consumer services manager for www.hpicheck.com, and follow a fall in such incidents over recent years.
Clocking a car means changing the mileage reading in order to dishonestly raise its value. The introduction of digital odometers has made the practice easier and "mileage correction firms" seem happy to change vehicles' mileage without asking questions.
Mr Teskey said research carried out between HPI and CAP in 2013 found that popular models like the VW Golf can double in value if they have 60,000 miles wound down.
Safety is the chief concern here, as well as finances he added, as a vehicle with more miles than the owner knows of could have components "excessively" worn, even if they look all right.
The cars will also probably have missed scheduled servicing.
Copyright Press Association 2014