I posted a response in ExPo's Excursion build thread, and am copying it here and modifying slightly due to recent decisions and research. This post has to do with measurements and decisions driving my future tire and wheel choice (37's).
There is a ton of discussion among various message boards about "lift required" to fit X size wheels, without real discussion on the backspace/offset.
I took some pictures and measurements this past weekend of my Excursion that is sitting on 285/75R17's (~34) on 17x8 4.5" backspaced wheels at full lock on both sides. I was planning on sketching up some lines to see what backspace/offset I needed to keep me off the leafs with a 37" setup.
I had someone holding the wheel at full lock to the right.
Here are some labeled pictures:
PS rear liner clearance:
PS leaf clearance (just hardly kisses spring at full lock):
DS front bumper clearances:
DS leaf spring clearance:
As you can see, the limiting factor is the rear of the tire touching the leaf spring at full lock. As you move up past 35's, you will need to trim the front bumper and pull the liner back. Neither of these things worried me. I also would like to get some measurements while the tire is full stuffed, I just didn't have the time or resources to get that done.
In order to get off of the leaf spring, you either need a high offset wheel or spacers (or a combination). I will likely get new wheels when I get the tires, and have chosen one with a -18 offset. My calculations show that I need a 1" spacer and this setup gets me completely off of the leaf spring.
I also plan to use a "zero rate add-a-leaf" on the front to push the axle 1" forward. This will get me off of the fenderwell and mean I only have to trim the bumper.
In the image below, 1 represents the dotted line that is the front bumper and 2 represents the dotted line that is the firewall fenderwell. The leaf spring is the rectangle on the left. The 4 highlighted lines are the newer 37x12.5 tire and the other, smaller rectangle is my current setup. This is at full steering lock:
In the image below, you can see that the new setup sticks ~2.4" past the fender (current setup is essentially in line). I wanted to check this in case I decide to buy some flares:
Obviously, I have made some assumptions and your mileage may vary. Hopefully some of these images and thoughts of mine can be useful to others as well.
Now, does someone want to lend me a money tree?