Here is a simple method: use a straight edge such as a metal ruler, place the straight edge on the outermost part of the rims where the wheel comes closest to the fender (preferably the top), then, measure from the outside wheel to the straight edge. This distance is the size spacer you will need to achieve the look that flushes or nearly flushes with the fender line. Too thin, you will still have wheel/caliper interference. Too thick, and your tires will hit the fenders. Use proper size of spacers not only ensure safety but also avoid breaking your local laws and regulations. Currently, BONOSS provides 15mm-110mm thickness wheel spacers to meet your needs.