Under- or over-inflation of tires can:
Eventually, you could have inconvenient flats or even dangerous blowouts. Over-inflation means the weight of your vehicle is not distributed evenly across the entire tread but is instead concentrated on a thin strip in the centre. Under-inflation means the tire is soft and the outside edges work harder than the softer centre. The extra rolling resistance makes your vehicle work harder to move and it can reduce gas mileage.
Be sure to check your tire pressure regularly and maintain the recommended pressure listed in your vehicle's owner's manual. You can usually find this recommended pressure on a sticker posted in the trunk, on the door pillar or in the glove compartment.
Each of your tires will wear differently due to its position and the type of vehicle you own. So tires will wear evenly, remember to balance and rotate your tires according to manufacturer's recommendations.