Clocking
Clocking is an illegal practice as old as the motor car, and involves using equipment to tamper with the odometer (the instrument that records the distance travelled) to make it look like a vehicle has a lower mileage than it really has. When performed, it is most often done to cars with a high mileage, and serves to artificially inflate the car's supposed value. A used car's asking price can be increased considerably for each thousand miles wiped from the odometer.
Modern cars with digital dashboards are not immune to this practice either; a computer, some leads and specially written software is all a clocker needs to turn back time, and without leaving any telltale evidence unlike conventional tampering.
The average car will travel about 12,000 miles every year, or a thousand miles per month, and so you can calculate a car's rough mileage if you know the age of the car. If an old and worn car has a very low mileage, it's pretty certain that it's been clocked.
If the used car you're looking at has a low mileage but is knocking on a bit, there are a number of things to keep an eye out for to check for possible clocking. Firstly, check the bumpers and the front of the bonnet for small chips. These are often made by stones during motorway travel, and could indicate a history of long journeys. Secondly, wear and tear on items such as the accelerator, brake and clutch pedal rubbers, a 'shiny' steering wheel, or worn out upholstery can show that the vehicle has had a lot of use, in line with a higher mileage vehicle. Conversely, if you know the car is an older model, but it's pedals, steering wheel and upholstery are new, this could be a sign that the seller is attempting to hide something.
For official mileage confirmation you can check the documentation that comes with the car. If the vehicle comes with past MoT certificates and service records then previously recorded mileage should be viewable, which you can check against the current mileage. If you are so inclined, you can get in touch with the mechanics or garages that dealt with the car servicing; they may be able to confirm what mileage was recorded when the work was undertaken.
You can use a service such as HPI Check to check previously recorded mileages of the vehicle. These services keep a record of all mileages recorded by MoTs and other services, so you can see exactly what their listed mileage was at certain points in the vehicle's life. Looking through this data, it should be easy to see any figures that don't fit in.
If you have access to the V5C certificate, you can also make note of the contact details of previous owners. Getting in touch with these previous keepers could get you the mileage that the car had when they sold it on to the next owner.
Should you still have doubts as to the legitimacy of a vehicle's mileage after you've performed these checks, we would recommend you try another car.