ilikemygtp, you are incorrect. The first number, in this case 235, is the width in milimeters if the tire. The second number is the % of the width that is the height from the outside of the rim to the outisde of the tire. So in the case of say 225/55, the width is 225 and the sidewall height is 55% of 225mm.
To answer your question, I would wouldn't go wider than 225 on a 6.5" rim. I have 235s on my 18x8s and I really do not think it would be a good fit on a 6.5 rim. I am going to go 245s on th enext set on mine.
ADDED:
Tirerack.com has an explantion here:
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=46
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04 Comp G - 3.4, alky, 1.9s, PT, 105# springs, OR rods, 85mm MAF, 39lbers