Improper inflation of tires affects gas mileage, tire wear and safety. To determine the proper inflation for your car's tires, consult the sticker located inside the glove box, on the driver's side door post or driver's side door, or inside the fuel filler door. This sticker will list the proper pounds per square inch of air pressure (PSI) for front and rear tires. Check your tire pressure with a pressure gauge when the tires are cold.
Poorer Handling
Over-inflated tires don't grip the road as well. While this may result in slightly better gas mileage, it also means less traction and poorer handling. Poor road conditions such as a wet or icy road magnify this problem, making it more likely that you'll have an accident if you're driving with over-inflated tires.
Poorer handling also translates to a rougher, less comfortable ride. When Popular Mechanics writer Ben Stewart over-inflated his tires for a gas mileage test, he reported the car's handling was compromised and the ride was rough and full of vibration. He also found no difference in gas mileage with the over-inflated tires.
Blowouts
Improper inflation, whether over-inflation or under-inflation, causes tires to overheat and increases the risk of blowout. A blown-out tire ruptures and can cause the driver to lose control of the vehicle. This could lead to a serious, even fatal accident.
Tire Wear
When tires are over-inflated, the tread wears out faster in the center of the tire. You'll need to replace the tires sooner, an expensive penalty for driving with over-inflated tires.
Over-inflated tires, being stiffer and more rigid, are also more susceptible to damage from hitting pot holes or striking curbs.