Too much pressure (over inflated)
The tyre 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 tyre in the centre. Over inflation gives less grip especially in wet or very cold conditions as the tyre tread is less able to grip and hold the road.
Too little pressure (under inflated)
This is the most dangerous scenario. The tyre becomes less rigid and offers inferior control. Eventually, the tyre casing will break, as the internal heat of the rubber rises rises due to lack of air to help support the shape of the tyre wall, this causes the tyre wall to alter shape excessively as the tyre rotates. The consequences can be severe such as a blow out at high speed.
Tread pattern will suffer, with premature wear on the shoulders of the tyre and extreme cornering force can even cause the tyre to come away from the rim again because there is insufficient air pressure to support the ideal shape of the tyre.
It is essential to check your tyre pressure regularly!