40mm is the stock offset on all our various 15x6 alloys which all fit a Rampage fine. But, offset alone only describes clearance/fitment if you're talking about the same 6" wheel width. If you were to go to a wider wheel with the same offset, say 15x7 +40, it would be half inch (~12.7mm) wider on each side of the hub flange so it would have ~12.7mm less clearance to the strut etc.
If you have a wheel that you know the specs of and know will fit the car, you can put those specs and the specs of the theoretical new wheel into an online calculator to see how the side to side clearance will change. If the diameter changes as well you have to account for those differences in the vertical plane yourself.
http://www.1010tires.com/Tools/Wheel-Offset-Calculator