Imagine if you had a wheel that was a 20×9 wheel. This means the wheel is a 20 inch wheel in diameter, and is 9 inches from each side where the tire beads on the wheel. This means the tire itself is 10 inches wide, as it does not include the flange on the front and flange on the back, each of which is about .5 inches.

Example 1.    A wheel with a 20×9 with a 0 mm offset (5 inch backspace)

This means the wheel is 10 inches wide, and the mounting pad is right in the middle of the wheel if you were to draw a line down the middle of it.

Example 2.    A wheel with a 20×9 with a -25 mm offset (4.5 inch backspace)

This means the wheel is 10 inches wide, and the mounting pad is 25 millimeters “offset” to the left of the mid line of the wheel, thus making the wheel come outbound more than a 0 offset.

Example 3.    A wheel with a 20×9 with a +25 mm offset (5.5 inch backspace)

This means the wheel is 10 inches wide, and the mounting pad is 25 millimeters “offset” to the right of the mid line of the wheel, thus making the wheel come inbound more than a 0 offset.