The punishment for tampering with a digital speedometer for a car depends on the jurisdiction. In some states, tampering with a digital speedometer for a car dealership is fined quite heavily at $10,000 or more per infraction, while at other dealerships the punishment is somewhat lighter but still severe. The dealership will be under the scrutiny of the attorney general’s office for a number of years following, and every transaction will be watched quite closely. Right down to the question of whether the dealership meets the requirements of the privacy laws. In a federal offense, the fine is 5 years and $100,000. Quite a stiff for a car that can be declared TMU (true mileage unknown) which will then allow the speedometer to be swapped out.
For more info, see How The Odometer Reading Works