Different measurement of the same thing.

 

Offset and BS are just two different ways of measuring the same thing.
As long as you don't go to a really wide tire, like over 12.50's, you should be fine with those. A wide tire would probably rub some on the control arms at full steering lock. The most commons setup is a 9 inch wide wheel with 4.5 inches of BS.

 

Back Spacing (BS) is the measure from the mounting surface (rim to axel flange) to the outside lip of the rim. (usually in inches)

Offset if the distance from the mounting surface to the center of the rim. (usually in milimeters)

Width of rim (outside of lip) = (BS - Offset - lip thickness) * 2
(note: convert 1 mm to 0.03937 inches)

Why is both rim width AND tire section width BOTH important? The most important measurement is the one rim measurements alone can't give you:
TOTAL BACKSPACING! (TBS)

TBS is the distance from the mounting surface to inside tire sidewall. In almost all cases, the tire width is greater than the rim width, so the tire is the first thing to rub.

Narrower wheels require a more NEGATIVE Offset to get the same effective TBS.


Example:
RANCHO 4" sport lift instructions states the lift kit was designed for BFG M/T T/A KM 35 x 12.50 x 18D on an 18 x 9 wheel with 4.5" Back Spacing. Total Back Spacing is 5.9".

A wheel with a width of 9" (bead to bead measurement) is actually closer to 10" over all. A 12.50 width tire has a typical section width of 12.5" causing 1.25" of tire over the rim.
4.5 + 1.25 = 5.75" (why this doesn't jive with 5.9" is beyond me).

If you choose to deviate from a lift manufacturer's recommended tire/wheel combination, at a minimum make sure you meet the recommended Total Back Spacing!

More information (with pictures):
Measuring Wheel Offset

 

I have 9" wide rims with 5" backspacing (Omm offset) and there is lots of room for 12.5" wide tires. No rubbing, no spacers, no problems whatsoever... My tires still stick out of the fenders a good inch and a half or so at least, plus I don't rub the front control arms any more. My previous rims were 5.71" backspacing and even those fit the JK with 12.5" wide tires, but with minor rubbing on the front control arms at full turn. Depending on your desired rim diameter or width, you may or may not need less backspacing.

My previous Moto Metal rims:5.71" backspacing...

On the Jeep...

Here I measured the backspace of the new Tracker rims... 5" it says...


Here is the front at full lock. I had my GF holding the wheel as far as it would go. You can see where my previous tire/rim set-up rubbed the lower control arm. Doesn't do it anymore...


At full turn, not touching the control arms...

A nice stance with the 5" backspacing and 12.5" wide Toyo's...I think they're perfect.


If you have any concerns with your rim selection, I would suggest talking to Discount Tire Direct. They really helped me out. They answered all my questions, reassured me even when I had doubt (I was initially concerned about the 5" backspacing myself) and set me up with the right ones.

 

The correct backspacing is contingent on the wheel width. For an 18x9 rim a BS of 5 to 4.5 is fine. THis translates to an offset of 0 to -12
FOr an 18x9 wheel
BS 4.5 = Offset -12
BS 4.75 = Offset of -6
BS 5 = Offswet 0

Most people will tell you that the max you can go is 4.75 and that 5 is too much and that you will rub on a 9 inch wide wheel. I think Runewolf1973 does a fantastic job dispelling that myth. I have 18x9 rims wearing 325/65 tires. Tey are not installed yet but as soon as they are I to will post pictures showing how they dont rub.

PS - a 5 BS on a 9wide rim will stick out past the fender the least. A 4.5 BS on a 9wide rim will stick out past the fender the most.