There are dishonest dealers out there who intentionally roll odometers back so they can artificially inflate the vehicle’s value. Vehicles that have a low mileage are priced higher. By omitting 35000-40000 miles, they can boost the vehicle’s value by $2000-$3000 or more.
There are two types of odometers: mechanical and digital. With mechanical odometers, the speedometer can be removed from the dashboard and the digits can be rolled back manually.
The second type is the digital odometer. Digital odometers were supposed to make it harder for criminals to tamper with them, but they have still managed to find a way to hack them.
Most vehicles store their digital odometer reading on the instrument cluster on a rewritable EEPROM chip. Once the chip is located, they will have it desoldered from the board and get it connected to a computer where its data can be downloaded and manipulated.
The odometer data are then edited to reflect a lower mileage, which is then re-soldered to the odometer board and put back into the car.