Here's what I found after Googling "tire ratio explanation":

Our friends at SEMA member company Discount Tire Direct (7333 E. Helm Dr., Scottsdale, AZ 85260-3123, 480-443-5621, www.tires.com) explain tire sizing on their website along with lots of other tire terminology. Section height, also known as the aspect ratio is figured on a percentage of the section width. A tire size of 205/60R15 would have a section width of 205 millimeters and a section height of 60 percent of 205 millimeters, or 123 millimeters. Section width is the linear distance between the outside of the sidewalls of an inflated tire without any load on it.

By applying the formula on our website, you get an equation that looks like this:

Aspect Ratio = 123 DIVIDED BY 205

Aspect Ratio = 0.6 or 60%


So using that formula a tire that is a 285/60R18 would be: 60% of 285 = 171mm. Aspect ratio = 171 divided by 285 = 0.6 or 60%

255/55R18 would be: 55% of 255 = 140.25mm. Aspect ratio = 140.25 divided by 255 = 0.55 or 55%.

What does all this mean? I have no freakin' clue. but there it is.

Here's a more graphic (not THAT kind of graphic) explanation:

http://www.sizes.com/home/automobile_tires.htm