Jack your vehicle up and support it with a good jackstand. Remove the wheel. Place a straightedge against the brakedrum. Measure from the inner edge of the straightedge to the inner fenderwell or anything that the wheel/tire might rub. Example: rubber snubber, shock, etc. Subtract 1" from the measurement. That will be the MAXIMUM
backspacing you can use. To figure out how wide a rim you can use: measure from the inner edge of the straightedge to the inner edge of the fender and subtract 1". Add this figure to the one you got for the backspacing and you will know how wide a rim you can fit in the wheelwell.