Offset vs Backspacing: How to Measure and Convert Them

Two measurements with different reference surfaces

Offset is a signed millimeter distance between the nominal centerline and the mounting face. Backspacing is the distance from that mounting face to the physical inside lip of the wheel. Listings in North America often express backspacing in inches. Both describe wheel geometry, but their references differ: one uses a centerline and the other uses an actual edge.

For the same physical wheel width, more positive offset means greater backspacing. For different widths, the relationship cannot be read from offset alone. A wider wheel can have more backspacing even when its offset is lower. Convert the dimensions into consistent units before comparing them.

Measure the wheel, not the mounted tire

With a wheel safely removed and its mounting face accessible, a straightedge across the rear wheel lip provides the physical edge reference. The perpendicular distance between that reference and the flat mounting pad is the backspacing. The straightedge must reference the wheel lip rather than a tire sidewall. Any straightedge thickness introduced by the chosen measuring arrangement must be accounted for.

Also identify whether a stated width is nominal bead-seat width or measured overall width. An 8-inch wheel designation is not evidence that the wheel measures 8 inches across its outer lips. Ask the wheel manufacturer for physical dimensions when a measurement cannot be taken reliably, particularly with irregular lip shapes or unusual mounting-pad geometry.

Convert using a measured overall width

If the lips are explicitly assumed symmetric about the nominal centerline, backspacing in millimeters equals overall width divided by two, plus offset. The inverse is offset equals backspacing minus half the overall width. The symmetry assumption matters: overall width alone does not locate the nominal centerline when the lip extensions differ.

Suppose an 8-inch nominal wheel is physically measured as 230 mm across its lips and its offset is +35 mm. Under the stated symmetric-lip assumption, backspacing is 230 ÷ 2 + 35 = 150 mm, or approximately 5.906 inches. Working backward, 150 − 230 ÷ 2 returns +35 mm offset. These invented measurement inputs illustrate the arithmetic; they are not dimensions for a particular wheel model.

Use a known inner-flange extension when available

A second method uses nominal half-width plus the measured inner-lip extension plus offset. For an 8-inch nominal wheel with a 12 mm inner extension and ET35, backspacing is 101.6 + 12 + 35 = 148.6 mm, about 5.850 inches. This method does not require the outer extension to equal the inner extension because the inner lip is located directly.

Mickey Thompson publishes a practical conversion convention that allows one extra inch over nominal width. That convention is not a universal measurement of every wheel. WheelLabTools requires explicit physical geometry rather than silently adding one inch. If you only know nominal width and ET, the wheel-position tool can calculate nominal inner reach; label that result accurately instead of calling it exact backspacing.

Common questions