Easiest way to explain it...
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Carefully lay the wheel on the ground, face down. The face of the wheel is the front, or the part that you see when the wheel is on the car.
Lay a straight edge across the inner bead of the wheel. Be sure that your straight edge is perfectly straight and true or your measurement will be off.
Place the end of the tape measure against the inner mounting flange of the wheel. This is the section of the wheel that sits against the brake drum or rotor.
Measure from the mounting flange to the bottom of the straight edge that is laying across the wheel on the bead or edge of the wheel. The resulting number is your backspace measurement.