Under-inflation is one of the greatest killers of tyres. Tyres
running at less than optimum pressures can suffer a 25% loss in
tread life due to increased shoulder wear. They are also dangerous.
Don't pump too much air into your tyres either. Gross
over-inflation causes your tyre to bulge out at the centre. Which
means that the centre of the tread will come into greater contact
with road surfaces than any other part of the tread. So, the tread
centre wears out earlier. Because an over-inflated tyre has a
smaller surface area in contact with the road than properly
inflated tyres, it suffers from reduced traction.
Over-inflation of tyres also increases the risk of bruise damage.
That happens when your tyre hits an object but you can't really
tell. Tyres which are grossly over-inflated also create undue
strain on your car's suspension.