Narrowing rear diff.

I ran 15" x 10" with 5.5" backspacing and endeded up narrowing about 2.5" inches so 1" less backspace means narrowing it 1" more, so 3.5" I would not take a guess at this-bad things can happen!!!!! I would recommend getting your tires/wheels mounted, set them up under the car where you want them to sit and measure inside rim mounting surface to opposite rim mounting surface measure several times, then call the Diff Dr., the man knows the spacing and figures in his head very nice to deal with and had my parts fast. hope this helps