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OK, I'm not sure if this has been adequately addressed or not -- trust me, I've done a ton of searching on here and can't seem to find a straight answer on this.
So, I'm looking at all possible 33" to 35" tire sizes and have determined that fitting a 35" tire on a 3/1 kit is not only possible but has been done numerous times.
What I fail to comprehend is how to determine what width of tire can be accommodated without rubbing on the UCA or body mounts or plastic on the fender-wells.
The stock wheel I have is an 18"x8" with a +60 offset. I'm not sure what the backspacing is on it (though I can find out if I take one off the truck I suppose).
I assumed that having rims with a +30 or +40 offset would move the wheel out by that amount minus 1/2 the extra width of the rim. So, on a 9" wide rim, a +40 offset would move the tire out 20mm - (1/2*(9inches-8inches=25.4mm)) = 20mm - 12.7mm = 7.3mm. Of course the wheel takes up an extra 12.7mm on each side, so, 12.7-7.3=5.4mm extra on the inside and 12.7+7.3=20mm on the outside.
So does having a lesser "positive" offset (or even a small negative) offset make you more prone to rubbing on the UCA and body mounts? Or is it the other way around? I keep seeing posts where I think it is one way, only to see other posts where it clearly claims the opposite. It just seems very confusing to me. I think if there was a way to determine the precise position of these items in relation to the wheel hub, we could more easily make a conclusion as to how wide a tire with what combination rim (backspacing and offset) will fit with or without certain modifications.
If we could make that determination, maybe we can create some kind of a sticky to illustrate how to measure via wheels/tires and that way people (like me) can be a little more comfortable ordering the proper size tires.
Some of this info can be derived from those that have posted their wheel/tire measurements and observed something like 1/4" space between UCAs and body mounts, etc. I hope this wasn't too confusing... I'm just trying to narrow down what I can do without having to order and send back different sets of tires... :)
Going from the +60mm offset to a +40mm offset pushes the wheel mounting surface 20mm out, plus the rim is 1/2" wider. You were on the right track with your calculations but the extra wheel width is the part that you divide in 1/2. 1/4" more backspacing and 1/4" outwards. Going from a 18x8mm +60 to a 18x9 +40mm your wheel will stick out 26.35mm (20mm plus 1/4") more or roughly an inch. So I guess to answer your question the more positive the offset the more the wheel will tuck into the fender, the more negative (or less positive) you go the further out the wheel sticks. Check this link on how to figure out wheel measurements.

https://www.rsracing.com/tech-wheel.html
 
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