1979-1993 Mustang Bellhousing Inspection Plate

The wheel size is the diameter of the wheel, in inches, not counting the tire. Modern tires in North America have several other measurements associated with their size. A tire designated as a "225/70R14" denotes a tire with the following dimensions: width of tread: 225 mm; ratio of tire height to width: 70 percent; rim diameter: 14". Therefore a 185/70R14 has the same rim diameter and aspect ratio, but a smaller overall diameter and narrower tread face than the 225/70R14.

The easiest way to measure backspace is to lay the wheel face down onto the ground so the backside of the wheel is facing up.

Take a straight edge and lay it diagonally across the inboard flange of the wheel. Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is the backspace.

The photo below shows three wheels with 2", 3", & 4" backspace.

The bolt pattern or bolt circle is the diameter of an imaginary circle formed by the centers of the wheel lugs. Bolt patterns can be 4, 5, 6, or 8 lug holes. A bolt circle of 4x100 would indicate a 4 lug pattern on a circle with a diameter of 100mm.

The diagram indicates the proper measuring methods. 6 lug wheels are measured like the 4 lug wheels.

The offset, measured in millimeters, can be negative or positive, and is the distance from the hub-mounting surface to the rim's true centerline. A positive offset means the hub-mounting surface is closer to the outside edge of the wheel; a negative offset means the hub-mounting surface is closer to the inside edge of the wheel. Offset affects the scrub radius of the steering and it is advisable to stay within the limits allowed by the vehicle manufacturer. If tires are fitted which are significantly wider than those specified by the manufacturer, a compromise may have to be adopted whereby a wheel with less offset is used to prevent the tire rubbing on the suspension.

The centerbore of the wheel is the opening in the middle of the wheel, surrounded by the lug holes. This opening helps to center the wheel to the hub. Its diameter is typically measured in millimeters (mm) and varies in size depending on vehicle application. Manufacturers approve up to a 1% variance in tolerance to pass wheels during inspection.