Maxima Wheel Offset
*This information is directly related to the 5th-Gen Maxima, but does offer a good base-level overview of offset.
For a 5th-Gen Maxima, anything <30mm offset may cause the tire to scrub the inner fender, so you may not want to get a rim with an offset lower than that. Others (member Fezzik for one) have successfully installed 20mm spacers on the rear with factory 45mm offset rims (equates to a 25mm offset) without scrubbing.
Above is a representation of two offsets, 45mm and 35mm. Offset is the distance from the mounting hub of the rim to the exact center of the rim. 35mm means the mounting section of the rim is 35mm out from the center of the rim. The black curved portion of the image represents the fender. Below shows offset in detail
The lower the mm, the less amount of offset from the rims center section. The lower the offset, the higher amount of rim width extends out from the hub.
If you went outside and looked at your car from the back, at an angle where you can see the rear and front rims and wheel wells, you will notice that the back rims seem to be set deeper into the wheel well than the fronts which makes the front wheels look like they stick out further. The factory offset is 45mm. You can make the back end better by replacing the rear with rims that have a 30mm offset, since more of the rim will extend outward. Of course, after doing this, you could never rotate the back rims to the front of the car (assuming the front still have a 45mm offset). If you replace all rims with 30mm offsets, the back rims look better, but the front rims now stick out even further. To get around this whole issue, you could get 45mm rims and use 15mm spacers (or more common 40mm offset rims with 10mm spacers). The spacers would be on the back to help resolve that "set in" look.
