When you over inflate your vehicles tires the tread and sidewall of the tires become stiffer.
The stiffness and over inflation of the tires causes the traction and performance to be reduced and cause the tires to wear out unevenly and much sooner.
Your fuel mileage also can decrease as a result of over inflated tires and it could be dangerous as well because the tires are not flexing properly and could blow out on the highway leading to a rollover or other accident.
Always make sure your tires are properly inflated and never over inflated and if your vehicles tires are over inflated be sure to let some of the air out to bring the tires to the proper air pressure.