Proper tire inflation pressure is probably the single most important factor in determining whether or not you receive maximum mileage out of your tires, and what sort of fuel mileage your car gets. Improperly inflated tires will wear incorrectly and more quickly than properly inflated tires. Tires that are improperly inflated can also have slight differences in revolutions per mile. An over-inflated tire is slightly taller than it should be, and needs to turn just a bit less than a properly inflated tire to cover a mile. This can cause problems with drive axles, especially if one tire is underinflated and the other is overinflated.
For more info, see How to Extend Your Tire Life