Driving on under-inflated or over-inflated tyres compromises any or all of the following.
Stopping distance: Properly inflated tyres optimize tread contact with the road, therefor providing enough contact patch for traction. Applying brakes with improperly inflated tyres takes more distance to stop the vehicle which could be catastrophic in an emergency situation.
Ride and handling: When tyres have excess air, they're overly rigid, which means they don't absorb as much impact, so the car rides rougher and suspension components wear faster. An overinflated tyre's tread may crown, allowing only the center portion to contact the pavement, which decreases traction. When the pressure is too low, the ride might be softer, but the tread may not meet the road uniformly.
Fuel economy: Underinflated tyres greatly reduce fuel economy because they assume a physical character; high rolling resistance, that requires more than the average power to maintain the same vehicle speed, which means using more fuel on a journey.
Treadwear: Along with the traction decrease that comes with overinflation, it also causes the tread to wear more quickly in the center. Underinflation causes wear on the tread shoulder and allows heat buildup, therefor speeding wear.
Load bearing: Tyres are rated to carry a maximum amount of weight at a prescribed tyre pressure. Some cars specify a higher pressure for greater loads. At best, a tyre that's underinflated for the load at hand will suffer the problems detailed above. Extra heat generated in the tyre can cause premature failure which could be prevented by higher but right pressure.
Miss-inflated tyres pose dangers to self and other road users. In addition, operating costs are higher due to rapid wear.
An illustration of different air pressure levels.
Image source; https://www.tyre-shopper.co.uk/tyre_pressures
- Tyre Plus. "Why Are Car Tyre Pressures Important?" Www.tyreplus.co.uk. Tyre Plus, n.d. Web. 30 May 2016. <http://www.tyreplus.co.uk/tyre-tips.asp>.