For clearance measuring, it is all about the total width (referred to as sectionial width) of the tire mounted /inflated on a particular rim.......not tread width. Most retailers (Summit /Jegs/ etc) have this spec listed.
Example....the 14" (tread) X 32" (diameter) x15" (rims) Firestone Drag 500s for the Flash mounted on 13" wide rims (for mph) have a sectional width of 16-3/4" .
I have 3/4" clearance to the inner wheel well with the above. It was running only about 1/4" with the 14W tire combination that was on it previously.....too close for me.