Fahad, you don't seem to understand how to read tire sizes so let me explain for you.
205/60R15
the first number, 205, is the width of the tire. A 205 is smaller than, say a 225. My VW has 175, a much skinnier tire
the second number 60, is the tires profile, or the height of the tire compared to it's width. A tire with a profile of 45 will look thinner around the wheel. When you get up to an 18" wheel, the profile numbers can get very small. That's where the tires look more like rubber bands than tires

The lower the profile, the better your handling, since the sidewall doesn't roll over.
the last number 15, is the diameter of your wheel or the size of the opening in the tire. You cannot fit a 205/60R15 tire on an 18" wheel, it is too small, also, a 18" tire won't fit a 15" wheel, it's too big.
So as you get bigger wheels, the profile number (60) on the tire will get smaller. If you get 17 or 18" wheels with 60 series tires, your speedometer will go out of whack. (If they even fit in the wheel wells)
Hope this was of some help! I recommend that if you have the money, get 17" wheels and tires from
http://www.tirerack.com Luke, from tirerack is on these boards every now and then, he will be more than willing to help you out!
