If offset is measured from centerline of wheel, a 8" wide rim with -12mm offset would sit further away from truck than a 9" wide rim with -12mm offset because it has half an inch more rim on either side of the center line. Am i overthinking and missing something simple?
Correct, unlike backspacing, the same offset on a wider rim, will not leave you with the same clearance as with a narrower rim.
I am currently running 15x8 wheels with -12mm offset. I am in the process of moving up to a 17x9 wheel but cannot get a straight answer on what offset to get. I would like to maintain the current tire to frame clearance that I have. So the conclusion i have come to is that I would need a 17x9 wheel with at least -18mm offset to equal my 15x8 with -12mm offset. Am I even in the ballpark?
Your current 8" rim with -12mm offset, has a 3.5" backspace, for the same backspace on the 9's, you need -25mm offset,