Too much pressure (over inflated)
The tire becomes hard and the contact area is reduced. Grip and performance is impaired especially in bad weather conditions and the steering becomes less assertive. Other symptoms include premature wear to the tire in the centre.
Too little pressure (under inflated)
This is the most dangerous scenario. The tire becomes less rigid and offers inferior control. Eventually, the tire casing will break or crack, as the internal heat rises due to lack of air pressure.
The consequences can be severe. Because of the lack of pressure to hold the sidewalls fairly rigid the extra flexing can cause the walls to become weaker and may ultimately fail as speed and cornering forces increase. Tread pattern will suffer, with premature wear on the shoulders of the tire and extreme cornering force can even cause the tire to become detached from the rim.