Under-inflation is the most common cause of tyre failures and may result in a “blow out”, with unexpected loss of vehicle control possibly leading to an accident. Source: Dunlop tyres care & proper inflation
An American University group have done some calculations to show us how various amounts of psi tyre underinflation have an exponential affect on wasted fuel and additional tyrewear:
National Tyres and Autocare in UK say "Incorrect tyre pressure can reduce your tyre life by up to 25% and increase your fuel consumption by as much as 5%"
Victorian Automobile chamber of Commerce (VACC) show why tyre pressure is crucial to car safety.
It may not be possible to look at a tyre and actually see under-inflation of 5 psi. However, under-inflation of 5 psi can reduce a tyre’s tread life by 25%. A typical tyre may also lose 1 to 2 psi a month, if not checked and adjusted. Source: Dunlop tyres care & proper inflation
Properly inflated tyres improve the steering, stopping and traction of your vehicle. Recent studies suggest under inflated tyres can increase stopping distance by close to 8 meters, which could in many cases be the difference between missing and hitting something on the road.
Ensuring your tyres are pumped to the recommended level will save up to 100 kilograms of greenhouse gas each year - Department of the Environment
The USA National Highway Traffic Safety Administration – (NHTSA) estimates that 49 of 79 deaths and 6,585 to 10,635 injuries could be prevented annually if all vehicles were equipped with tyre pressure monitoring systems.
Following the Tread Act of 2005, NHTSA stipulated that from 2008, all newly manufactured or imported US cars (including all passenger vehicles and light trucks) will be fitted with TPMS.
Similarly in March 2009, the European Union approved legislation measures imposing mandatory adoption of Tyre Pressure monitoring Systems, with an implementation period of two years, starting November 2012.