Rim width and offsets?

[mudNrust85]

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?

[jimbo74]

wouldn't you want the same offset than?

honestly, the -18mm should be fine though, you are talking about maybe 1/4"more

[mudNrust85]

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?

[emsvitil]

Unless the rim is going to hit something (like the brake caliper) you need the tire widths.

Tire rack will have the specs at given rim width.    Say it's 10.5 on 7.5" rim.    Then the rule of thumb is add .2" for each .5" increase in rim width ( or subtract).       

With this, you can see where the side of the tire is.

[Snowtoy]

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,