Under-inflation occurs when there is lack of air pressure in the tyres, causing the tyres to deflate. Under-inflation might be caused by lack of tyre maintenance, tyre punctures, or any damages done to the tyre.
When there is under-inflation, the tread of the tyres will come into more contact with the ground. Tyre deflection will increase as there will be excessive flexing of tyre treads and the sidewalls of tyres. More internal heat will be produced and there will be more rolling resistance. The tyre treads will wear out faster, especially near the sides of the tyres.
This will affect the precision of vehicle steering, the stability of the car, and also decrease the fuel efficiency due to increased drag.
Over-inflation happens when there is excessive air pressure in the tyres, making the tyres stiff and unyielding. The cause of over-inflation usually is when too much air is pumped into the vehicle tyres.
There will be uneven wear on the tyres, with the centre of the tread wearing out faster than the other sections. In extreme cases, the tyres might blow out after a long drive or if they’re too hot.
Over-inflation will reduce traction and grip of the tyres. This is dangerous especially on wet surfaces, where your control over your tyres and vehicle will be reduced. It will also be a more harsh and jarring ride for you and the passengers as the tyres are harder and do not respond well to any irregularities on the ground.
Also, over-inflated tyres will be more vulnerable to damages while driving. For example, when they come into contact with debris or solid objects. This drastically reduces the lifespan of your tyres.