I have what may be a very stupid question about tires, but I am going to ask it anyway, Jalop cred be damned.

Stupid Tires Question

Here's the situation: My wife drives a 2007 GTI that has the 18" potato cutters (pictured), and correspondingly small sidewall. She is ... well... not the most attentive driver, and has a bit of a problem with hitting curbs. Not surprisingly, all of her wheels have curb rash on them, and a few of them are actually bent. As a result of the bent wheels, I recently picked up a used set of wheels for her car off of Craigslist. The new wheels came with old, worn out tires on them, which I will be replacing before putting them on her car. However, in looking at those tires, I noticed that the sidewalls extend out from the edge of the rim noticeably more than the tires that are currently on her car. The tires on my wife's car are nearly flush with the wheel (meaning that if you hit a curb, it is guaranteed curb rash) but on the new set of tires, there is actually a significant amount of tire that sticks out beyond the side of the wheel (meaning that if you were to hit a curb with those tires, there is at least some hope that you wont get curb rash). However, both sets of tires are the same size (225/40R18).

So the question is this: Is there some tire size measurement that I am missing here, regarding the width of the sidewall? Or do some manufacturers just make slightly wider tires (or thicker sidewalls)? When I get new tires, I would definitely like to get ones that offer a bit more protection to the wheels, but I am not sure how to go about finding which tires those are.