What is wheel offset?

Discount Tire defines offset as the distance from the mounting surface to the wheel centerline, measured in millimeters. Positive offset places the mounting surface toward the outside face of the wheel. Zero offset places the mounting surface directly on the centerline.

Negative offset places the mounting surface toward the back, or inner side, of the wheel. The diagrams show these mounting-face positions relative to the centerline. Discount Tire notes that negative-offset wheels are sometimes called deep dish rims.

Offset measures from the mounting surface to the centerline. Backspacing measures from the mounting surface to the wheel’s back lip, according to Discount Tire, and is expressed in inches. At the same wheel width, positive offset adds backspacing and negative offset reduces it.

To measure backspacing, Discount Tire describes laying the wheel face down, placing a flat item across it, and measuring from the mounting surface to the underside of that item. Summit Racing Help Center specifies that the straightedge must contact the metal rim, not the tire. Les Schwab says wheel width must be considered along with offset, especially when the replacement wheel is wider.

The wheel offset calculator uses this formula: backspacing (in) = (wheel width + 1 in) / 2 + offset (mm) / 25.4. Enter nominal wheel width in inches and retain the positive or negative sign on the offset.

The extra 1 in represents the rim flanges, about 0.5 in on each side, which nominal wheel width excludes. This is a formula-based estimate; the wheel maker’s spec sheet is the reference for a particular wheel. The rim size explanation helps distinguish nominal width from overall width.

• By the formula, an 8 in wheel at +35 mm offset has 5.88 in (149 mm) backspacing.
• By the formula, an 8 in wheel at 0 mm offset has 4.50 in (114 mm) backspacing.
• By the formula, a 9 in wheel at −12 mm offset has 4.53 in (115 mm) backspacing.

At the same wheel width, lowering offset moves the wheel outward; raising offset moves it inward. Discount Tire describes positive offset as tucking the assembly farther under the fender, while zero or negative offset moves it toward or potentially past the fender. That outward projection is often described as poke.

Width changes the result too. By the formulas, changing from a 17x8 +35 wheel to a 17x9 −12 wheel moves the outer edge 2.35 in (59.7 mm) outward. The inner edge moves 1.35 in (34.3 mm) away from the suspension.

Les Schwab warns that too much positive offset can cause inner-edge rubbing against bodywork or suspension and interference with brake parts. It also warns that too much negative offset can increase steering-wheel kickback, add suspension stress and cause poor handling. Fit depends on the vehicle, wheels and suspension; an edge-position calculation alone does not establish compatibility.