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Discussion Starter · #1 ·
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... :)
 

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Discussion Starter · #5 ·
Thanks for posts guys! My question isn't so much about how to calculate the offset or backspacing or even the relationship of that with the original wheels (regular or TRD). I'm more concerned with finding a way to calculate or measure the distances between the items that may rub (i.e. body mounts, UCAs, skid plate, plastic trim in fender well, etc.) and the wheel hub. That way, for example, if somebody says that they are going to put in a 35" wheel that is 12.8" wide on a rim with a 6.5" backspace and a +12mm offset, we can say something to the effect that "the wheel/tire combo will fit with a 3/1 lift, but you'll need to cut part of the plastic off the front fender well, trim/bend the skid plate a 1/2" and remove the mud flaps" or we could say something like "it will fit, but you'll be 1/4" from the UCA with that combination... if your rims had a +X offset instead, you would be a good 3/4" away from the UCA" or something to that effect.
My goal for this is to come up with a formula for being able to tell what possible issues will be present with certain wheel/tire combinations so a lot of these types of issues can be avoided before ordering the tires/wheels or having to try on lots of different sets.
I'll probably need to figure a way to the truck up on the lift, but also get more input into the common types of "modifications" required to make a tire/wheel combo fit. So far, to start off with, the most common things I've seen that need to be altered are:
  • front plastic trim in fender-well
  • rear-plastic trim in fender-well or mud flap
  • body-mount cutting/grinding and re-welding
  • bending the skid plate
As I understand it, I haven't seen examples of somebody modifying the UCA to accommodate the wheel/tire combo, but if somebody has and would like to chime in on how to do it or whether it was even worth doing, I'd be all for adding that to the list. Also, there are probably other things that I'm missing from the list, but I would certainly like to grow it as much as possible.
 

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Discussion Starter · #6 ·
I gotta give this a bump...
Trying to see if there is some mathematical way to make a determination about whether or not a certain size tire/wheel combination will fit with/without lifts. All help is appreciated!

P.S. - I've seen the numerous threads on wheels/tires including the infamous 35" tire on 3" lift. I'm trying to find out myself whether a 325/60r20 will fit on a 3" lift with a +40mm offset (don't know what the backspace is, but will determine that when I receive the rims) but not sure since it is literally a 35.4" tire. But more importantly, if there is a way to devise what items need to be considered for clearance/rubbing issues and if it could be measured in such a way as to help determine what tires/wheels would work, that would be even better! (Probably save a lot of multiple tire/wheel posts)
 

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Discussion Starter · #8 ·
Yeah, maybe it was too ambitious of a goal, but I know it could have certainly helped a lot of people. It is just frustrating having to go through this trial-and-error-type method... but I guess that is what its looking to be...
 
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