Backspacing is a measurement system used to determine where the wheels and tires are placed when bolted onto a vehicles hub mounting surface. It is the distance from the back of the mounting pad to the back lip of the wheel. Backspacing is usually measured in inches. The higher the backspace, the more the wheel is pushed inboard towards the cars centerline.

Wheel offset is the distance from the mounting surface to the centerline of the wheel, expressed in millimeters. Positive offset means that the wheels are pushed inboard towards the centerline of the car, while negative offset means that the wheels are moved outboard towards the fenders, widening the track.

Backspacing and wheel offset are essentially the same thing, but they use different methods and measurements to determine how far inward or outward tires will sit on a vehicle. Proper offset and backspacing measurements are important to ensure that the new wheel and tire package has enough clearance so that nothing rubs against the suspension, brakes, or vehicle body. It is also important for driving safety, as the wrong offset can reduce vehicle stability or interfere with braking.