Randy, did you try looking at these (Last link on bottom)
Here's some links not only for you, but any others that have Q's about wheels and tires...
Offset
Reading the side wall
Tire Calculator (I like this calulator and am going to put it on this site, just in case the link goes bad..)
I did some checking and the Nitto's
285/50/18 approved rim size is 8.0 in to 10.0. Their tread width is 11.54 and diam is 29.06 sidewall 5.6.
275/60/18 that is approved for 7.5in to 9.5. Their tread width is 11.14 and diam is 31.18 sidewall 6.5.
255/55/18 (closest thing to stock) that is approved for 7.0 in to 9.0. Their tread width is 10.33 with a diam of 28.94 sidewall 5.5
The stock 18's on a 7.0 in rim are 9.646 in wide and a diam of 29.6 the sidewall is 5.8in.
When time comes I may go with the 275/60/18's they'll fill up the well better.. Just depends on how much I need to trim. The only problem that I saw with the Terra Grapplers 285/60R18 120S 31.57 height 10.67 width 8.0-(8.5)-10.0 rim is that 8.0 inch rim size.. I think that you would get too much sidewall roll with a rim that is 1 inch more narrow defeating the "On-Road" performance.. Plus, I'm not sure if it wouldn't look like a balloon.
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