Speedometers were put in all motor vehicles since 1910 not only for vehicle drivers to know how fast they are going so they can brag about it. More importantly speedometers are there to help prevent road accidents because the driver can see if he or she is driving too fast.
This is why it is very important to make sure that your speedometer is well calibrated such as it displays your current speed accurately. Not only will avoid getting speeding tickets with an accurate speedometer, your passengers, and the vehicles and people around are also safe because you are aware of how fast you are traveling.