The offset of a wheel is the distance from its hub mounting surface to the center line of the wheel. The whole point of an offset is for the end user of the wheel to know how much in inches (4+3), or in the case of millimeter offset, how many mm the wheel will stick out or tuck in from the mounting surface of the hub. Remember, even a couple of millimeters can affect performance, ride quality, the look/stance of your vehicle and safety. It takes precise measurement to make sure aftermarket wheels and tires fit properly in the wheel well, without rubbing against the fender or vehicle components.
Two important measurements that describe wheel’s proper position are offset and backspacing. Offset measures the distance between the wheel’s mounting surface and center line, while backspacing measures the distance between the wheel’s mounting surface and innermost lip. These measurements help determine which wheels are compatible with which vehicles, so you will need to be aware of them if you decide to install non-OEM wheels.
To properly understand wheel offset, you must first know the basic characteristics of a wheel assembly. The offset describes the wheel’s ideal position, so that it can freely rotate without rubbing against the fender or the brakes.
A wheel’s offset is measured using the distance between its center line and the mounting surface. Millimeters are used as the standard unit to describe offset, which can be positive, zero, or negative.
- Zero offset means that the wheel’s mounting surface lines up with the center line.
- Positive offset refers to a mounting surface extending past the center line and closer to the face of the wheel.
- Negative offset describes a mounting surface situated closer to the back of the wheel.
Every vehicle has a specific offset. Modern front wheel drive vehicles usually have positive offset, while older or customized vehicles may have a negative offset. Most wheels have their correct offset data embossed on the mounting surface or on a spoke. Check out the diagrams below for a visual illustration of each offset type.
The mounting surface extends past the wheel’s center line, closer to the face of the wheel. Most factory rims, with wide low profile tires, like the Challenger, have this offset type.
The mounting surface is centered within the rim, aligned with the wheel’s centerline.
The mounting surface sits before the wheel’s center line, closer to the back of the wheel and the car’s suspension system. Many classic musclecars and modified cars have this "deep-dish" set-up.
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 backspace. Backspacing, similar to offset, is the distance from the hub mounting surface to the inside lip of the wheel (measured in inches).
HOW TO MEASURE A WHEEL OFFSET
To calculate the wheel offset, you will need the following measurements:
Wheel backspace (see above).
Wheel center line (outboard flange to inboard flange measurement, divided by 2).
Once you have these measurements, you need to subtract the wheel center line from wheel backspace to get the offset.
If backspace is less than the wheel center line, the offset is negative.
if backspace is greater than the wheel center line, the offset is positive.
To convert inches to millimeters, multiply inches by 25.4.
To convert millimeters to inches, divide mm by 25.4.
The following is a handy custom wheel offset chart for wheel widths, from 5.5 to 15 inches.