And if ya want to see what a 15x10 with 3.625 bs looks like with 35x12.50's. Check my test fit I just did on a procomp rim.

https://www.jk-forum.com/forums/show...highlight=8179

 

Here's another reference...






18x9's with 3.5" of B.S.

 

This is how I always understood:

The backspacing (distance from the bolts on the outer side of the wheel to the center of the wheel) only needs to be smaller (pushing the wheel more away from the car) when you want to put wider wheels on rims with an unchanged width.

With other words for a the standard rim is 7 inch wide with a 10 inch wide tire and 5 3/4 backspacing with for example a wheel offset of ET 44. This implies that the tire sticks out of the rim 1 1/2 inch on each side. If you have 1/2 inch clearance without rubbing, this would imply that if you put 12 1/2 inch wide tires that they stich out 1 1/2" + 1 1/4" or 2 3/4" on both sides of the rim... and they would rub. Therefor you need to get lower backspacing (by installing a spacer)

If you put wider rims that 'grow' with the tire with, the ET value will become smaller if you keep the same backspacing.... since the center of the wheel will also move moreout side.
As an example on the vehicle above with 7" wide standard rims and 10" tires you can put 12 1/2" tires on rims that are 9 1/2" wide (even 9" would work since you have a spare 1/2" clearance). In this case your backspace can stay the same, but the Offset will be closer to 0 (12mm).

I find the image below is a bit confusing since it shows a negative offset

 

Actually, you are incorrect, that would be a positive offset like the factory JK wheel. Negative goes the other way. See chart below.



My wheels are 18x9 with 3.5" of backspaing, that translates into a -38 offset. Factory Jk wheels are 17x7.5" (Rubicon) with 6.25" of B.S. or roughly a +25 offset.

 

Ok now I am confused. The JK Wrangler indeed has a positive offset.
But in the top image the backspacing is smaller < than half the rim width... and on the JK Wheel the backspacing is higher > than half therim width.

I think that in the image below the outside of the rim is on the left and not on the right as in the above representation.

What do you guys think?

 

Yes, if the offset is towards the face of the wheel or outside the center line, it's a positive offset or high backspacing, offset inside of the center line or towards the back of the wheel is a negative offset or lower backspacing.

Does that make sense?