Wheel offset and backspace measure the same thing but in a little different way.The offset describes the wheel’s ideal position, so that it can freely rotate without rubbing against the fender or the brakes.

What is offset?

Offset is a measurement in millimeters from the backside of the mounting plate to the centerline of the wheel.

  • A positive number puts the mounting plate further away from the backside of the rim.
  • A negative measurement moves it closer to the backside. Since offset is based on the center line, the width does factor in.
  • A zero offset when the mounting pad position is in line with the position of the centerline.

That is to say an 8-inch wide +45mm offset and 7-inch wide +45mm offset results in a 1/2" difference in wheel positioning. The 8-inch wide will be further into the wheel well. 

What is Backspace?

Backspacing is an older system of measurement to determine how deep the mounting pad is located in the wheel. Backspace measures the distance from the mounting plate to the back edge of the wheel. The higher the backspace the further into the fender well it will sit. 

  • More backspace indicates that the wheel protrudes further into the wheel well and closer to the suspension parts — which increases the risk of rubbing
  • Less backspace indicates that the wheel protrudes less into the wheel well and therefore reduces the risk of rubbing
  • Lifted trucks and SUVs often have a maximum allowable backspace pre-specified by the lift kit manufacturer. With too much backspace, your wheel or tire won’t fit your lifted truck or SUV
  • Few trucks accept more than 5” backspace. Less than 5” backspace is more common on lifted trucks, but confirm with your lift kit manufacturer to be sure
  • Check with your installer prior to purchasing your wheels to ensure you are ordering the proper backspace for the look and clearance you need

Here is the formula to calculate the Offset and Backspace

ET=Offset unit = mm

B=Backspace unit = Inch

J=Wheel Width

If you know the offset , to work out the Backspace (B)

(ET / 25.44) + (J/2) + 0.5 = B

If you know the Backspace, to work out the Offset (ET)

(B-0.5-(J/2) ) x 25.44 = ET


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