There are two things to consider, wheel backspace and offset backspace, To find the offset, start by determining the wheels' centerline. To obtain that, measure the wheel's overall width from one outside edge to the other. This is not always the same as the width given in the manufacturer specs.

For example, a width of 15x8 inches refers to the distance between the inner flange and outer flange. Next, divide the width in inches by 2 to find the centerline. If the actual width of our example wheel is 9 inches, its centerline would be at 4.50 inches. Then, subtract the centerline to get offset 9 - 4.5 = 4.5.

1: Measure the room in your wheel wells relative to the wheel hub.
2: Backspace = The distance from back of the wheel to the wheel's mounting space, in inches.
3: Centerline = The wheel width divided by 2, in inches.
4: Offset = centerline - backspace, which will be positive or negative, in inches.
5: Offset in Millimeters = Offset x 25.4 (they use mm in their literature)

So...measure your wheel, then measure the new one and you will know exactly how it will fit. In your post you say you are going from a 15x8 with 5.5 backspace to a 15x7 with 4.5 backspace, if those numbers are correct you will only gain half inch.