Because you are trying to fit for a maximum width tire...as has been mentioned, you must consider the entire assembly of components you're using. Obviously different rims, rotors or drums will stack-up differently. Also, the width rim you're using will alter the sidewall. And the sidewall is what you'll need to engineer clearence for. I would suggest error on the narrow side because, you may add spacers and/or you may upgrade to a alloy hat in the future. Sidebar-Sledge made me a good deal on these! TY