Ok, time for some simple math.
My truck started out with 8" rims with 3.5" of backspace for a total of 72"
I removed the 8's and replaced with 10's with 2" of backspace, I used a calculator so my math should be correct.
By Removing 3" total from my backspace, I added 3" of track width (Tires stick out 1.5" on either side = 1.5" of gained track width)
So, 72 + 3 = 75 right? So if I am at 79" total, where did that other 4" come from? Oh yeah, the 2" on either side from the added width of the rim for a total of (your still with me right?) 79" of total track width.
This has nothing to do with tread width............. This is simple math, and you are more than welcome to stop by and check my measurements, I have both sets of rims and tires.