The x9 and x10 is the width of the wheel in inches.
325 is the width of your tire in MM, which means your tires are 12.8 inches wide.
60 is the aspect ratio of your tire in percent. The aspect ratio is what tells you the height of the tire. 60 means the aspect ratio is 60% of the width of the tire, which is 325 MM. Doing the math 325 x .6, you get 195 MM, which is 7.8 inches. That 7.8 inches is the distance from the outer edge of the wheel to the tread of the tire. So, adding 7.8 + 18 + 7.8, you get 33.6 inches, which is the height of your tire.
18 is the wheel diameter in inches.
325/60R18 tire size should fit a 9-12" wide wheel, so it should fit both of the sizes you have listed. The best and fastest way to determine what size wheels will work is to look up the tires you plan to use. Most, if not all tire manufacturers, will tell you what width wheels will work with their tire sizes.
Last edited by Vecte; 04-02-2012 at 04:40 PM.