Why is it illegal to alter the odometer on a vehicle

Odometer reading is a vital tool in car assessment, it represents safety, real age of the vehicle, and many other general factors. The act of altering the number constitutes forgery and fraud, due to it's apparent nature. For example, if it would be legal to alter the odometer reading, then many people could buy brand new cars at their local dealerships, with reality being that those are used cars with thousands of miles on them, and used car owners will be more prone to be in car collisions, without having a clue of the real state of their car.