[Jerry Bransford]

but they dont use the backspacing method (i never heard of backspacing till i got a jeep).. most use offset off the center of the rim which is measured in positive or negative mm

Offset is common for many cars and most imports but trucks and Jeep wheels use backspacing as their key dimension since it's a more useful dimension where clearance is concerned. You won't find offset used on offroad oriented Jeep wheels.