If they are double banded offset isn't changed.
Single banded on the front will bring the offset down by however wide the band is. (25.4mm to an inch is a handy conversion to know when working out offsets.)
Changing a 5" et49 wheel to a 9" wide wheel using a 4" band on the lip of the wheel changes the offset to et-1.8
(Wheels will sit 4" further out on the outside edge but no change will be made to where they sit on the inside edge)
Changing a 5" et49 wheel to a 9" wide wheel using a 2" band on the lip and a 2" band on the inside of the wheel keeps the wheels at et49
(Wheels will sit 2" further out on the outside edge and 2" further in on the inside edge)

