[Barry45RPM]

Very simply put, "18 inch" refers to the rim or wheel size (height without tires mounted on them) , or the inner hole diameter of your tires. 8.5 and 9 refers to the width.

There are other tire dimension factors too, such as the profile... low profile/high profile, which refers to the distance from the edge of the rim to the outer edge of the tire, commonly thought of as the "tire wall"... the part with all the writing on it.

Your car has low profile tires, which means you should try to avoid bad pavement/pot holes, as there is not alot of tire there to absorb the bump, and wheel damage can occur.

Since these are not "Race Cars" designed for ultra high performance in day to day driving, the staggered wheel set up (different sizes front to rear) is more "sizzle" than steak, although they do give you a bigger foorprint in the rear. All 4 the same size makes more sense Dollar wise to most drivers. Such is the price of "Sportiness".