Wheel sizing is a simple matter once you understand the different wheel sizing labels and terms and the methods of measuring wheel dimensions. Our team of experts at RENT A WHEEL | RENT A TIRE make these calculations daily to provide our customers the best fitment for their vehicles.
Rim diameter is the diameter of the rim’s bead seat floor, not the diameter of the outer rim edge. The rim edge, or lip, extends beyond the bead seat area to capture the bead and prevent the bead from slipping off the rim. If we say that a wheel has a 19-inch diameter, we mean that the diameter of the bead seat floor measures 19 inches. The outer rim edge diameter may actually measure 22 inches or so, depending on the design.
Rim width is the distance from the outboard bead seat to the inboard bead seat. Just as with rim diameter, rim width is not measured at the overall width of the wheel (you only should be concerned with the seating width that the wheel offers for the tire beads).
If a wheel is said to be 10 inches wide, the distance between the bead seats is 10 inches. The overall width, measured between the rim outside edges, is immaterial.
Backspace is the distance from the rear face of the wheel center (the face that contacts the hub) to the rim’s rear outer edge. In this case, we don’t care about the location of the bead seat. Instead, we do want to know how far the outer edge of the rim extends from the center section’s hub contact face.
Offset refers to the location of the center section\’s hub contact face, relative to the mid-point of the rim width. The mid-point of the rim width is exactly that: If the rim is 10 inches wide from bead seat to bead seat, the mid-point, or halfway mark, would be 5 inches from either bead seat.