I've run them as tight as a 1/4 of an inch to the leaf spring. Since the leafs move up and down with the tire through road irregularities, and is somewhat removed from sidewall flex during cornering, I never had any contact issues there. Spacing to the wheel lip and inner wheel well need to be bigger because of the same suspension articulation. The amount of space needed there would depend on how widely you exercise the suspension. Low sitting cars that have to traverse bumps and driveways at an angle may need more room than a higher sitting car that can go over things straight up. At these points, 1/2 to 1 inch is probably closer to the norm to avoid contact. But again, we get back to that whole "how big is the sidewall bulge" issue as well as how hard do you use it.
