The EPA estimates that we can improve our gas mileage by up to 3.3 percent by keeping our car's tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by 0.3 percent for every 1-psi drop in pressure of all four tires.   At today's gas prices, this is the equivalent of saving $.10 on every gallon of gasoline purchased!

Improperly inflated tires will also wear out quickly and may cause a accident. Statistics and research on transportation indicate that due to tire under-inflation, at least 260,000 accidents occur per year, and at least half a million gallons of fuel are wasted each year. Overinflated tires are also risky as heat causes air to expand, increasing the likelihood of a blowout. Overinflated tires will increase wear in the center of your tire tread, reducing tire life. Too much air pressure can also decrease ride quality by making bumps and road imperfections more apparent.
 

Source: Green Garage Associates