You can determine by checking the sidewalls of the wheel. The first 3 numbers on the sidewall represent the width of the tire in millimeters. The next two represent the aspect ratio, that is, the ratio between the height and the width of the tire. The letter that follows represents the diameter of the wheel. The next numbers represent the amount of weight the tire can carry. The last letter represents the speed at which the tire should move. http://4wheelonline.com/Ion_Alloy_Wheels.169706