the offset a wheel is the measurment from the centerline of the wheel to the bolt-up face of the wheel. the centerline of the wheel is the exact center of the wheel when measuring the width. so if the wheel is nine inches wide, the center line of the wheel would split in into two 4.5 inches wide sections, the inner rim and the outer rim. the inner rim is the part of the wheel closest to the suspension, and the outer rim is the half closest to the fender. backspacing is the measurment from the the inner rim of the wheel to the bolt-up face.
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