^ called it. Wheel width is measured by inches so your xxr's are 8 inches wide. Offset designates how far the mounting hub of the wheel is spaced away from the geometrical middle of the wheel. So 20 mm from the middle of the wheel. Negative offset then starts spacing your mounting point closer to the back side of the wheel. The offset will not change the width of the wheel but it will push the wheel closer to the outside of the fender, the lower offset you go. You may have just not noticed.