Here's some pics that should help you see the difference. The first 4 pics are the Nitto Terra Grappler, they have very square shoulder design....meaning the shoulder is closer to a 90 degree angle (tread are to side area).
By contrast, look at the Yokohoma Geolander AT/S and you can see the shoulder is more rounded.

If you think of that tire turning, you'll see that one of the first areas where a tire is going to rub something within the wheel well is the tire shoulder area. The shoulder is the tread blocks on the outer most ring of the tire. The NTGs have quite square outer tread blocks, and thus, a square shoulder.


Also, here's a really helpful tire/wheel size calculator that shows a grea t visual of the tire/rims size, offset, etc...
http://www.rimsntires.com/specs.jsp

Plug in your OEM specs (245/60-18), then your proposed size. You'll need to get "OFFSET" number for this to be valid. The RED center wheel in the diagram shows you how offset affects how far out the rim is specific to your proposed rim. If you think about it, the further out the tires are, the more likely they are to hit the cladding on the swing arc (when turning). For reference out OEM rims are +38 offset.












Here's some visual help. These are my Cooper Zeon LTZ 285/60-18s front driver's tire turning, you can see where the "shoulder" would be the first to rub the cladding on the swing arc. Thees tires have fairly square shoulders, but not as much as the NTGs.


So, to solve that, I cut a small triangle shaped piece from the FRONT, OUTER section, and that's all it took to prevent the rubbing. It's VERY VERY easy to cut the plastic cladding, especially in a straight line. A sawzall works, and so does a Dremel.



There really is a ton of threads on this topic, you might try the search function and see if you can dig up some of them for reference.
HTH!
Good luck to you.