by offset do you mean backspacing? The stock backspacing is 5.25". To increase the width of the stance, you decrease the backspacing. ie 4.5", 3.75", etc.

Jon

 

Offset, or Backspacing is the measuremtent from the inside of the wheel mounting surface to the edge of the inside rim. Thus a wheel with 0 inches of backspaceing would be perfectly flat on the back, and put your wheels way outboard. (Kind of like the silly little CRX moonbuggies that the city kids drive around here) Though you'd probably find yourself getting very good at replacing wheel bearings with wheels like that. 4.5" is what most people seem to run with slightly larger tires. Any more, and the leverage (particularly with larger tires) that the wheel will put on your bearings and your steering U-joints will get pretty ugly.

 

I've been trying to figure this one out too. From what I've heard, stock alloy wheels have 4.5 backspacing on a 7 inch wide wheel. That puts your stock front tires about 1 inch inside the fender flares.

Wider tires like 31's will extend inward and outward from the wheels. Therefore, about .5 inch inside the fender flares.

Most aftermarket wheels are 8 inch wide. If you get 4.5 inch backspacing, the inside of the wheel will be in the same place, but the outside of the wheel will be an inch farther out. 31's on these will be about even with your flares. 32's will be .5 inch past the flares.

Go to 4 inch backspacing, and your 31's are .5 inch outside the top edge of the flares. 3.5 backspacing of course pushes them out farther.

Some stores do measure by "standard offset, positive and negative." Standard to them means that on an 8 inch wheel, the mounting point is dead center at 4 inch. Then they measure positve or negative from there. Its a little confusing, but if you have to, bring a tape measure.

Randimal
"I have a mind like a steel....uh....animal catcher thingee...."

 

For some reason, I seem to recall stock offset being 5.25 inches... but I'm not positive... Someone else could probably tell ya better. I do know that 4.5 puts the wheels further outboard that stock.

 

My bad. I just went out and measured and it looked to be around 5.5. Mine are stock 15x7 alloys. The standard 5 spoke with the Jeep logo.

Everything else in the explanation still sounds correct.

At this point, I would prolly recommend 15x8 with 4.5 for most applications. Rim will be an inch farther from control arms (for fat tires) and tires will stick out up to two inches farther depending on width. (May need a good lift to stuff)

Randimal
"I have a mind like a steel....uh....animal catcher thingee...."

 

Just to clear everything up about offset and backspacing here goes. Backspacing is the distance between the inner most edge of the rim and the wheel mounting surface of the wheel. Offset is the distance between the center line(on a 10 inch rim the center line is 5 inches from the street side and 5 inches from the inner side) of the rim and the inner wheel mounting surface. Definitely not the same thing but they are closely related.
Trystan