Backspace and offset are two ways of describing where the wheel sits in relationship to the hub. As such, there is a relationship between the two making – it possible to convert from one to the other.
Backspacing is the easiest to understand. It is simply the distance from the back of the mounting pad to the back lip of the wheel. It is measured by laying the wheel on its face, placing a straight edge across the wheel, and then measuring down to mounting face. The higher this measurement the more the wheel is pushed inboard towards the car’s centerline, as shown in the illustration above.
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 like a car with front wheel drive. Negative means that the wheels are moved outboard towards the fenders, widening the track.
TO CONVERT OFFSET TO BACKSPACING:
For positive offset wheels: (Wheel Width +1)/2 + (offset * .03937)
For negative offset wheels: (Wheel Width +1)/2 – (offset * .03937)
TO CONVERT BACKSPACING TO OFFSET:
Backspacing – (Wheel Width + 1)/2 * 25.4 = offset
Round this result to the nearest milimeter. If the answer is a negative number the wheel has negative offset, if positive then a positive offset.
This is Mickey Thompson DRAG / STREET Technical Bulletin #7.