The custom rear fenders we carry work well with both 16" and 18" vintage style tires. While the rim size varies, the outside diameter (O.D.) of these tires doesn't vary drastically. When mounting a custom fender, the distance mounted from the tire will make the radius match better or worse. A convenient way to keep even spacing all the way around is to use electrical tape (or similar) to hold a piece of hose, old motorcycle chain, or other piece of material to the tire and set the fender on it. This will keep the gap between tire and fender even throughout. The material used varies based on the thickness, which determines the desired gap.