Here's a good visual interactive chart:
http://www.rimsntires.com/specs.jsp
OEM size on the 16" is 245/70-16, +38 offset
OEM size on the 18" is 245/60-18, +38 offset
Play around with the interactive chart and see how the dfferent numbers change the sizing.
The offset is important because it moves the wheel farther outward, or farther inward. If you think about how that would affect the FRONT wheels when turning.
I just bought some -2 rims, so they will be sticking out further. When the fronts turn, you can see how they'll hit the fender well so I have to get the wheel well trimmed a fair amount.
Keep in mind the width of the rim (you're looking at 9", OEM is 7") and again think of how a wider rim would affect the turning on the fronts....and then offset added onto to that formula, and you can see how the rubbing will be come an issue. The wheel wells can be , um....modified...with some cutting/trimming, and some good hammering to move the one bolt farther back.
There's a bit more to it, so I'm sure one of the others will give you more info.
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