255 is the width of the tire in millimeters. 45 is the sidewall height in millimeters, ZR is the speed rating 149+, and 17 is the size rim the tire will fit on. 285-40/ZR18 front and 315-35/ZR18 rear is pretty much the widest tire you can fit under the car. You'll have to roll the inside of the fenders though. Or take out some bolt I hear for the front's. It doen't change the appearance or hurt the car.