It’s all about the math… here is how I did this. I hope you can follow. I wanted to keep proper backspacing AND understand where the tire fits in relation to the fender flares.
First, stock wheel info is here:
https://www.jeepforum.com/forum/f9/stock-tj-specifications-452871/#post4121478
I have the Grizzly
15x7
5 x 4.5” bolt pattern
Backspacing 5.5”
Offset = + 38mm
I also had P225 / 75R15 tires.
Width = (225mm to inches =) 8.8”
Then I draw a picture:
"A" is the distance I need to understand. This is essentially the back spacing, plus the distance the tire protrudes from the rim.
A = + 38mm (offset) + Half the width of the tire (4.4”)
A = 1.5” (+38mm) + 4.4” = 5.9”
A= 5.9”
So, I wanted to upgrade to 31 x 10.5” tires. Value A should not exceed 5.9”.
10.5” (tire width) / 2 = 5.25”
New tire is 5.25 subtract that from old tire 4.4 = 0.85” difference
To maintain A, then I need to shift the offset -0.85” or -21mm
My old offset is +38mm subtract the shift of -21mm means I need a tire with a +17mm Offset. To maintain the "A" distance.
The OTHER factor, is your fender flares. The more I push the tire away from the jeep, the more it sticks out past the fender flares.
If my old jeep wheels / tires were flush with the fender flares, then I increased my tire width (half of it) by 0.85” AND I changed the offset by 0.85” for a total of 1.7”.
So now, my wheels stick out past the fender flare by 1.7”
So, where did I end up at? Based on availability and price, I purchased a 15 x 8” wheel with a -12.7mm Offset
So my new A dimension is 5.25” + (-12.7mm) = 4.75” So my new A = 4.75”, which is less than 5.9”, so I know I have plenty of inside clearance.
But it also, shifted my tires out further past the fender flares. I increased my tire width (half of it) by 0.85” AND I changed the offset from +38 to -12.7 = 50.7mm (~2 inches) 0.85” + 2” = 2.85”
So my new tires stick out past my fender flares about 2.85”