Here's a very cool calculator, useful to see how much a rim will stick out, (or in), more than the OEM rims.
You only need to know the width and offset of the rim under consideration, (OEM rim values are shown in the Note, below).
http://www.americanracing.com/techcenter/OffsetCalculator/offset.html

Note:
The Off road pkg. seven spokers are 17" x 7.5" with a +30mm offset, approximately 5.5" backspacing.
Non-off road SE and LE rims are 18" x 8" with a +25mm offset, approximately 5.5" backspacing.

The attachments shows an example of the input and output of the calculator.
The first attached thumbnail show the OEM 7.5" rim as the current wheel and the OEM 8" as the new wheel. Notice the results, in the second thumbnail. The inside clearance is only 1mm different ... both rims basically have the same backspacing.

If you have the OEM 8" rim, its numbers would be entered into the current wheel and the rim you are considering, (for example, maybe an 8.5" rim with +18mm offset), would be entered into the new wheel field.

Attached Thumbnails